perf-investigation-logger

Append structured investigation notes to {state-dir}/perf/investigations/<id>.md.

Follow docs/perf-requirements.md as the canonical contract.

Required Content

1. Exact user quotes (verbatim)
2. Phase summary
3. Decisions and rationale
4. Evidence pointers (files, metrics, commands)

Output Format

## <Phase Name> - <YYYY-MM-DD>

**User Quote:** "<exact quote>"