feat(review): add hit/miss telemetry to the repo-culture-profile cache (#4509)

[JSONbored]

Summary

• readCachedCultureProfile's cache (a signal_snapshots-backed TTL, plus a merged-PR-count drift invalidation) works correctly — but its hit/miss branch was completely unlabeled, unlike every sibling cache in this codebase (ai_review, ai_slop, linked_issue_satisfaction, miner_detection), one of the six capability gaps issue feat(stats): AI-feature cache/reuse-efficiency rate, tracked over time, public #4448 identified.
• Adds an incr()/recordAuditEvent pair on both the hit path (inside extractRepoCultureProfile, right where a non-null cached result is returned) and the miss/re-derive path, mirroring the ai_review cache's instrumentation exactly (event names follow the same github_app.X_cache_hit/X_cache_miss convention, metric names follow gittensory_X_cache_hit_total/_miss_total).
• The single hit/miss branch point correctly counts the merged-PR-count drift-invalidation path as a miss too, since readCachedCultureProfile's null return already unifies every invalidation reason (no snapshot, TTL expiry, drift, malformed row) — there's no separate code path to accidentally leave uninstrumented.
• Purely observability — no functional change to caching behavior, confirmed by the pre-existing 24 cache/invalidation tests passing unchanged.

Closes #4509.

Scope

• The PR title follows type(scope): short summary Conventional Commit format.
• This PR is focused and does not mix unrelated backend, UI, MCP, docs, dependency, and deploy changes.
• This follows CONTRIBUTING.md and does not reintroduce GitHub Pages, VitePress, site/, or CNAME.
• I linked a currently open issue this PR resolves (Closes #4509).

Validation

• git diff --check
• npm run actionlint
• npm run typecheck
• npm run test:coverage locally — src/review/repo-culture-profile.ts is 100% statements/branches/functions/lines in isolation (confirmed via a scoped coverage run), and the precise diff-line cross-reference against the full-suite lcov.info shows zero uncovered lines/branches.
• npm run test:workers
• npm run build:mcp
• npm run test:mcp-pack
• npm run ui:openapi:check
• npm run ui:lint
• npm run ui:typecheck
• npm run ui:build
• npm audit --audit-level=moderate
• New or changed behavior has unit/integration tests for new branches, fallback paths, and sanitizer boundaries — 4 new tests: a hit invariant (fires only the hit pair), a miss invariant (TTL expiry, fires only the miss pair), a regression test for the drift-invalidation path specifically (confirms it's counted as a miss, not silently uninstrumented), and a fail-safe test (a failing audit-event write on the hit path is swallowed, never surfacing to the caller) — all empirically verified to fail against a reverted (uninstrumented) implementation and pass against the real one.

Safety

• No secrets, wallet details, hotkeys, coldkeys, user PATs, private keys, raw trust scores, private rankings, or private maintainer evidence are exposed.
• Public GitHub text stays sanitized, low-noise, and does not imply compensation guarantees or optimization tactics.
• Auth, cookie, CORS, GitHub App, Cloudflare, or session changes include negative-path tests. (N/A — no such surface touched.)
• API/OpenAPI/MCP behavior is updated and tested where needed. (N/A — no API/OpenAPI/MCP surface changed.)
• UI changes use live API data or real empty/error/loading states, not production mock/demo fallbacks. (N/A — no UI changes.)
• Visible UI changes include a UI Evidence section below. (N/A — backend-only observability change, no visible UI.)
• Public docs/changelogs are updated where needed; changelogs are only edited for release-prep PRs. (N/A)

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.09%. Comparing base (6646b87) to head (78eb11f).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4601   +/-   ##
=======================================
  Coverage   94.09%   94.09%           
=======================================
  Files         430      430           
  Lines       38215    38222    +7     
  Branches    13931    13931           
=======================================
+ Hits        35958    35965    +7     
  Misses       1600     1600           
  Partials      657      657           

Files with missing lines	Coverage Δ
src/review/repo-culture-profile.ts	100.00% <100.00%> (ø)

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