[codex] Order get_context recent docs by date

[yibeichan]

Summary

• order get_context recent docs by frontmatter date instead of ingest-time updated_at
• add id ASC as a deterministic tie-breaker
• add regression coverage where all docs share the same updated_at but have different dates

Root cause

The MCP get_context path sorted recent docs by docs.updated_at, but ingest does not populate updated_at from markdown. A rebuild inserts all rows in one transaction, so updated_at ties and the recent list becomes arbitrary.

Validation

• npm test -- --testPathPatterns=get-context-tool.test.ts
• npm test

Closes #194