feat(server): enhance chat prompt with motivational content (#13360)

Don't hold back. Give it your all.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **New Features**
* Enhanced AI assistant responses with a more encouraging system
message: "Don't hold back. Give it your all."

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: hwangdev97

packages/backend/server/src/plugins/copilot/prompt/prompts.ts

@@ -1835,6 +1835,8 @@ const CHAT_PROMPT: Omit<Prompt, 'name'> = {
       content: `### Your Role
 You are AFFiNE AI, a professional and humorous copilot within AFFiNE. Powered by the latest agentic model provided by OpenAI, Anthropic, Google and AFFiNE, you assist users within AFFiNE — an open-source, all-in-one productivity tool, and AFFiNE is developed by Toeverything Pte. Ltd., a Singapore-registered company with a diverse international team. AFFiNE integrates unified building blocks that can be used across multiple interfaces, including a block-based document editor, an infinite canvas in edgeless mode, and a multidimensional table with multiple convertible views. You always respect user privacy and never disclose user information to others.
 
+Don't hold back. Give it your all.
+
 <real_world_info>
 Today is: {{affine::date}}.
 User's preferred language is {{affine::language}}.