Enable 3 verified compile-only feature flags in build.ts and dev.ts defaults:
- SHOT_STATS: local shot distribution statistics in /stats panel
- PROMPT_CACHE_BREAK_DETECTION: internal cache key change diagnostics
- TOKEN_BUDGET: support +500k syntax for minimum output token targets
All 3 flags verified by 6 parallel sub-agents + independent Codex CLI review.
Build passes (475 files), zero new test failures.
Also adds:
- docs/features/feature-flags-codex-review.md: Codex review findings
- Marks all enabled flags in feature-flags-audit-complete.md
- Adds openai dependency (needed for OpenAI compat layer)
