docs: add comprehensive project documentation in Chinese

[lightjake]

Add full project documentation covering:

• docs/README.md: project overview, tech stack, quick start guide
• docs/architecture.md: system architecture, layer responsibilities, data flow
• docs/concepts.md: core concepts (SSE, A2A protocol, HITL, event system, etc.)
• docs/notes.md: usage notes, configuration, caveats
• docs/backend/core/: orchestrator, planner, super agent, agent connections, conversation management, task management
• docs/backend/adapters/: asset data adapters (YFinance, AKShare, AdapterManager)
• docs/backend/server/: HTTP server API routes, schemas, DB layer
• docs/backend/agents/: Research Agent, Auto Trading Agent, AI Hedge Fund agents
• docs/frontend/pages/: frontend pages and routing
• docs/frontend/agent-chat/: chat interface components and renderers
• docs/frontend/state/: state management (Zustand), SSE client, API layer
• docs/examples/: end-to-end usage examples

https://claude.ai/code/session_01PXAkfeJgDnDzdeT3hUCzXX

📝 Pull Request Template

1. Related Issue

Closes # (issue number)

2. Type of Change (select one)

Type of Change: Bug Fix / New Feature / Code Refactor / Documentation Update / Other: __________

3. Description

Please describe the changes made and why they are necessary.

4. Testing

• I have tested this locally.
• I have updated or added relevant tests.

5. Checklist

• I have read the Code of Conduct
• I have followed the Contributing Guidelines
• My changes follow the project's coding style