docs: dogfood report for v3.9.4 (#988)

* docs: dogfood report for v3.9.4

Comprehensive dogfooding session results: 6 issues filed (#979-#984),
3 fix PRs submitted (#985-#987). Assessment: 6.5/10. Report covers
cold-start, command sweep, rebuild/staleness, engine parity, benchmarks,
release-specific tests, MCP, programmatic API, and config.

* docs(dogfood): fix bug count and incremental delta prose (#988)

- Correct '~249' prose in section 8 to reflect two-stage behavior
  (first rebuild +211, subsequent +249).
- Fix 'Three other' to 'Four other' in section 12 — #980/#981/#982/#984
  are four distinct bugs.

Author: carlos-alm