feat(mcp): add Hy3 code review server (#3)

[Smily2333]

Summary

Implements the code review scenario for #3 as a local stdio MCP server powered by Hy3.

MCP tools

• review_diff: evidence-backed findings with severity and locations
• explain_changes: purpose, behavior changes, affected areas, risks, and reviewer focus
• generate_test_plan: prioritized regression and boundary test cases

Delivery

• OpenAI-compatible Hy3 API integration with validated structured outputs
• bounded inputs, retry/error normalization, and prompt-injection boundaries
• Python + uv packaging and one-command installation
• WorkBuddy and Codex configuration examples; optional Cursor compatibility
• bilingual README, architecture, client setup, demo, and validation documentation

Validation

• 21 pytest tests, including MCP stdio -> fake Hy3 API end-to-end coverage
• Ruff lint and format checks
• wheel and source distribution build
• MCP Inspector discovers all three tools
• live Hy3 calls for all three tools
• WorkBuddy end-to-end review_diff call
• Codex end-to-end generate_test_plan call
• two-client demo recordings, combined duration under two minutes

Demo evidence

• WorkBuddy review_diff demo
• Codex generate_test_plan demo

Closes #3