[docs][all] initialize AI coding harness docs

[VinCinx]

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docs

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[docs][all] initialize AI coding harness with AGENTS.md and architecture docs

• This PR title match the format: $$&lt;type&gt;$$$$<scope>$$ <description>
• The description of this PR title is user-oriented and clear enough for others to understand.
• Add documentation if the current PR requires user awareness at the usage level.
• This PR is written in English.

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[docs][all] 初始化 AI 编码辅助文档体系（AGENTS.md 及架构文档）

(Optional) More detailed description for this PR

en:
Add the AI coding harness documentation to help AI assistants (e.g., Claude Code, Cursor) understand and navigate this repository effectively. This includes:

• AGENTS.md: top-level entry point with project overview, core constraints, dev workflow, and common commands
• ARCHITECTURE.md: system architecture, code map, and dependency invariants
• ai-package.json: AI package metadata
• docs/AGENTS.md: mid-level index linking reference and guidance docs
• docs/reference/backend-modules-api.md: backend 6 DDD modules responsibility and API routes
• docs/reference/frontend-packages.md: frontend 59 packages layered structure
• docs/guidance/deployment-guide.md: Docker Compose / Helm deployment guide
• docs/guidance/idl-codegen-guide.md: IDL change and codegen workflow

zh(optional):
为仓库添加 AI 编码辅助文档体系，帮助 AI 编码助手高效理解和导航本仓库。包括项目入口文档 AGENTS.md、架构文档 ARCHITECTURE.md、后端模块 API 参考、前端包分层参考、部署指南和 IDL 代码生成指南。

(Optional) Which issue(s) this PR fixes

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