docs: Add AGENTS.md (#3461)

Create AGENTS.md

Author: gaby

AGENTS.md

@@ -0,0 +1,21 @@
+# AGENTS.md
+
+## Startup script
+
+- Fetch dependencies:
+
+  ```bash
+  go get ./...
+  ```
+
+- Install CLI tools referenced in Makefile:
+
+  ```bash
+  go install gotest.tools/gotestsum@latest                 # test runner
+  go install golang.org/x/vuln/cmd/govulncheck@latest      # vulnerability scanner
+  go install mvdan.cc/gofumpt@latest                       # code formatter
+  go install github.com/tinylib/msgp@latest                # msgp codegen
+  go install github.com/vburenin/ifacemaker@975a95966976eeb2d4365a7fb236e274c54da64c  # interface impls
+  go install github.com/dkorunic/betteralign/cmd/betteralign@latest  # struct alignment
+  go mod tidy                                              # clean up go.mod & go.sum
+  ```