chore: make contributor tooling work; fix drifted docs (#648)

- mypy.ini parses again (multi-line exclude and inline value comments made
  mypy reject the file); the mypy pre-commit hook is manual-only until the
  ~500 existing errors in src/ are paid down, and CONTRIBUTING says so
- .gitignore: ignore all of config/ except the templates (ytm_auth.json and
  others weren't ignored)
- .gitattributes: LF for .sh and .service
- claude-code-review: skip fork PRs, which have no secrets
- check_system_compatibility.sh: 3.13 supported, <3.10 an error
- docs/scripts drift: emulator guide, README API Metrics, route count,
  docs index, scripts README; pyflakes nits in dev scripts

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/claude-code-review.yml

@@ -6,6 +6,10 @@ on:
 
 jobs:
   claude-review:
+    # Pull requests from forks get no repository secrets, so without this
+    # guard every outside contributor's PR showed this check red for a reason
+    # they can't fix. Skipped checks don't block merging.
+    if: github.event.pull_request.head.repo.full_name == github.repository
     runs-on: ubuntu-latest
     permissions:
       contents: read