fix(skill): use registered Exa search tools

[LeonChaoX]

Summary

• replace the deprecated get_code_context_exa example with the registered web_search_exa tool
• direct exact repository/code lookup to Agent Reach's GitHub workflow
• add a packaged-skill contract test that rejects unavailable Exa tool calls

Root cause

Exa deprecated get_code_context_exa and no longer registers it by default, but the bundled search reference still instructed agents to call it. This caused an otherwise valid research workflow to fail at tool selection.

User impact

Generated skill guidance now uses tools available in a default Exa MCP setup and routes exact code search to the existing GitHub integration.

Validation

• python -m pytest tests/test_v2ex_channel.py tests/test_doctor.py tests/test_channel_contracts.py tests/test_skill_command.py -q — 37 passed
• python -m ruff check agent_reach/channels/v2ex.py tests/test_v2ex_channel.py tests/test_skill_command.py
• git diff --check

Closes #523

[Panniantong]

感谢修正 Exa 工具契约。你的原始作者提交已保留在 merged #577 中，并已通过完整 CI 与真实 Exa smoke，因此关闭原 PR。