chore(internal): add new package for tsdown build

internal/build/README.md

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+# LangChain Build System
+
+A modern build system for LangChain JavaScript/TypeScript packages that provides fast compilation, type checking, automated secret management, and advanced code generation for monorepo workspaces.
+
+## Overview
+
+This build system is designed to handle the complex requirements of LangChain's multi-package monorepo. It automatically discovers packages in the workspace, compiles them with optimal settings, and includes specialized tooling for LangChain's security patterns and dynamic loading capabilities.
+
+### Key Features
+
+- 🚀 **Fast Compilation**: Uses [tsdown](https://github.com/privatenumber/tsdown) for high-performance TypeScript bundling with Rolldown
+- 📦 **Monorepo Aware**: Automatically discovers and builds all non-private packages in yarn workspaces
+- 🔍 **Secret Management**: Built-in scanning and validation of LangChain's `lc_secrets` patterns
+- 📝 **Type Generation**: Generates both ESM and CommonJS outputs with TypeScript declarations
+- ✅ **Quality Checks**: Integrated type checking with [arethetypeswrong](https://github.com/arethetypeswrong/arethetypeswrong), [publint](https://github.com/bluwy/publint), and unused dependency detection
+- 🗺️ **Import Maps**: Automatic generation of import maps for convenient bulk imports
+- 📋 **Import Constants**: Dynamic detection and export of optional dependency entrypoints
+- 🎯 **Selective Building**: Build all packages or target specific ones with flexible filtering
+- 👀 **Watch Mode**: Real-time compilation with file watching capabilities
+- 🛠️ **Rich CLI**: Full-featured command-line interface with comprehensive options
+
+## Architecture
+
+The build system consists of:
+
+```
+infra/build/
+├── index.ts              # Main build orchestrator
+├── cli.ts                # Command-line interface
+├── types.ts              # TypeScript type definitions
+├── utils.ts              # Utility functions
+├── plugins/
+│   ├── README.md         # Plugin documentation
+│   ├── lc-secrets.ts     # LangChain secrets scanning plugin
+│   ├── import-map.ts     # Import map generation plugin
+│   └── import-constants.ts # Import constants generation plugin
+├── package.json          # Build system dependencies
+└── README.md             # This documentation
+```
+
+### Core Technologies
+
+- **[tsdown](https://github.com/privatenumber/tsdown)** - Fast TypeScript bundler with Rolldown
+- **[TypeScript Compiler API](https://github.com/microsoft/TypeScript)** - For source code analysis and type checking
+- **[yarn workspaces](https://yarnpkg.com/features/workspaces)** - For monorepo package discovery
+- **[unplugin-unused](https://github.com/unplugin/unplugin-unused)** - For unused dependency detection
+- **Node.js built-ins** - File system operations and process management
+
+## Usage
+
+### CLI Commands
+
+```bash
+# Get help
+yarn build:new --help
+
+# Build all packages in the workspace
+yarn build:new
+
+# Build with watch mode for development
+yarn build:new --watch
+
+# Build specific packages
+yarn build:new @langchain/core
+yarn build:new @langchain/core langchain @langchain/openai
+
+# Exclude packages from build
+yarn build:new --exclude @langchain/community
+yarn build:new -e @langchain/aws -e @langchain/openai
+
+# Skip various build steps
+yarn build:new --no-emit          # Skip type declarations
+yarn build:new --skip-unused      # Skip unused dependency check
+yarn build:new --skip-clean       # Skip cleaning build directory
+yarn build:new --skip-sourcemap   # Skip sourcemap generation
+```
+
+## Development
+
+### Adding New Packages
+
+1. Create package directory under appropriate workspace
+2. Add `package.json` with proper exports field
+3. Add `tsconfig.json` extending workspace config
+4. Run build - it will be automatically discovered
+
+### package.json Requirements
+
+Each package must have a properly configured `exports` field that includes an `input` property to tell the build system which source file to compile for each entrypoint:
+
+```json
+{
+  "name": "@langchain/example",
+  "exports": {
+    ".": {
+      "input": "./src/index.ts", // ← Required: Source file for this entrypoint
+      "import": "./dist/index.js",
+      "require": "./dist/index.cjs",
+      "types": "./dist/index.d.ts"
+    },
+    "./tools": {
+      "input": "./src/tools/index.ts", // ← Required: Source file for tools entrypoint
+      "import": "./dist/tools/index.js",
+      "require": "./dist/tools/index.cjs",
+      "types": "./dist/tools/index.d.ts"
+    }
+  }
+}
+```
+
+**Important**: The `input` property is required for the build system to understand which TypeScript source file should be compiled for each export. Without this property, the entrypoint will be ignored during build.

internal/build/cli.ts

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+#!/usr/bin/env node
+import { parseArgs } from "node:util";
+import { compilePackages } from "./index.js";
+import type { CompilePackageOptions } from "./types.js";
+
+/**
+ * CLI configuration with descriptions for auto-generated help
+ */
+const cliName = "yarn build:new";
+const cliConfig = {
+  name: cliName,
+  description:
+    "CLI program for compiling or watching packages in the repository",
+  options: {
+    watch: {
+      type: "boolean" as const,
+      short: "w",
+      default: false,
+      description: "Watch for changes and recompile automatically",
+    },
+    help: {
+      type: "boolean" as const,
+      short: "h",
+      default: false,
+      description: "Show this help message",
+    },
+    exclude: {
+      type: "string" as const,
+      short: "e",
+      multiple: true,
+      description:
+        "Exclude packages from the build (can be specified multiple times)",
+    },
+    noEmit: {
+      type: "boolean" as const,
+      default: false,
+      description: "Skip emitting type declarations",
+    },
+    skipUnused: {
+      type: "boolean" as const,
+      default: false,
+      description: "Skip unused dependency check on packages",
+    },
+    skipClean: {
+      type: "boolean" as const,
+      default: false,
+      description: "Skip cleaning the build directory",
+    },
+    skipSourcemap: {
+      type: "boolean" as const,
+      default: false,
+      description:
+        "Skip generating sourcemaps (e.g. `.map` and `.d.ts.map` files)",
+    },
+  },
+  /**
+   * only supported in later node versions
+   */
+  positionals: {
+    name: "package-query...",
+    description:
+      " Optional queries to filter packages (e.g., package name patterns)\n                   Multiple queries can be provided and will be processed together",
+  },
+  examples: [
+    { command: cliName, description: "Compile all packages" },
+    {
+      command: `${cliName} --watch`,
+      description: "Watch and recompile all packages",
+    },
+    {
+      command: `${cliName} langchain`,
+      description: 'Compile packages matching "langchain"',
+    },
+    {
+      command: `${cliName} langchain core`,
+      description: 'Compile packages matching "langchain" or "core"',
+    },
+    {
+      command: `${cliName} --no-emit`,
+      description: "Compile all packages without emitting type declarations",
+    },
+    {
+      command: `${cliName} --watch core openai`,
+      description: 'Watch packages matching "core" or "openai"',
+    },
+    {
+      command: `${cliName} --exclude langchain-community`,
+      description: "Compile all packages except langchain-community",
+    },
+    {
+      command: `${cliName} --exclude langchain-community --exclude langchain-aws`,
+      description:
+        "Compile all packages except langchain-community and langchain-aws",
+    },
+    {
+      command: `${cliName} -e community -e aws langchain`,
+      description:
+        'Compile packages matching "langchain" but exclude those matching "community" or "aws"',
+    },
+  ],
+};
+
+/**
+ * Generate help text from CLI configuration
+ */
+function generateHelp(config: typeof cliConfig): string {
+  const lines: string[] = [];
+
+  lines.push(`Usage: ${config.name} [options] [${config.positionals.name}]`);
+  lines.push("");
+  lines.push("Options:");
+
+  Object.entries(config.options).forEach(([key, option]) => {
+    const shortFlag =
+      "short" in option && option.short ? `-${option.short}, ` : "    ";
+    const longFlag = `--${key}`;
+    const padding = " ".repeat(Math.max(0, 15 - longFlag.length));
+    lines.push(
+      `  ${shortFlag}${longFlag}${padding}${option.description}${
+        "default" in option && option.default
+          ? ` (default: ${option.default})`
+          : ""
+      }`
+    );
+  });
+
+  lines.push("");
+  lines.push("Arguments:");
+  lines.push(
+    `  ${config.positionals.name.padEnd(15)}${config.positionals.description}`
+  );
+
+  lines.push("");
+  lines.push("Examples:");
+  config.examples.forEach((example) => {
+    lines.push(`  # ${example.description}`);
+    lines.push(`  ${example.command}`);
+    lines.push("");
+  });
+
+  lines.push(
+    `Copyright © ${new Date().getFullYear()} LangChain, Inc. All rights reserved.`
+  );
+  return lines.join("\n");
+}
+
+/**
+ * CLI program for compiling or watching packages in the repository
+ */
+async function main() {
+  const { values, positionals } = parseArgs({
+    args: process.argv.slice(2),
+    options: cliConfig.options,
+    allowPositionals: true,
+  });
+
+  if (values.help) {
+    console.log(generateHelp(cliConfig));
+    process.exit(0);
+  }
+
+  const packageQuery = positionals;
+  const watch = values.watch;
+  const noEmit = values.noEmit;
+  const skipUnused = values.skipUnused;
+  const skipClean = values.skipClean;
+  const skipSourcemap = values.skipSourcemap;
+  const exclude = Array.isArray(values.exclude)
+    ? values.exclude
+    : values.exclude
+    ? [values.exclude]
+    : [];
+
+  const opts: CompilePackageOptions = {
+    packageQuery,
+    watch,
+    exclude,
+    noEmit,
+    skipUnused,
+    skipClean,
+    skipSourcemap,
+  };
+
+  try {
+    const packageLabel =
+      packageQuery.length > 0
+        ? `packages matching: ${packageQuery.map((q) => `"${q}"`).join(", ")}`
+        : "all packages";
+    console.log(`${watch ? "Watching" : "Compiling"} ${packageLabel}...`);
+    if (exclude.length > 0) {
+      console.log(`Excluding: ${exclude.map((q) => `"${q}"`).join(", ")}`);
+    }
+    await compilePackages(opts);
+
+    if (!watch) {
+      console.log("✅ Compilation completed successfully!");
+    }
+  } catch (error) {
+    console.error("❌ Compilation failed:", error);
+    process.exit(1);
+  }
+}
+
+// Handle uncaught errors
+process.on("uncaughtException", (error) => {
+  console.error("❌ Uncaught exception:", error);
+  process.exit(1);
+});
+
+process.on("unhandledRejection", (reason) => {
+  console.error("❌ Unhandled rejection:", reason);
+  process.exit(1);
+});
+
+// Handle graceful shutdown for watch mode
+process.on("SIGINT", () => {
+  console.log("\n👋 Gracefully shutting down...");
+  process.exit(0);
+});
+
+process.on("SIGTERM", () => {
+  console.log("\n👋 Gracefully shutting down...");
+  process.exit(0);
+});
+
+main().catch((error) => {
+  console.error("❌ CLI execution failed:", error);
+  process.exit(1);
+});