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>

CONTRIBUTING.md

@@ -58,10 +58,13 @@ integration tests.
 3. **Keep PRs focused.** One conceptual change per PR. If you find
    adjacent bugs while working, fix them in a separate PR.
 4. **Follow the existing code style.** The pre-commit hooks run
-   `flake8` (E9, F63, F7, F82 plus bugbear `B` checks), `mypy` on
-   `src/`, `bandit`, and `gitleaks` — install the CLI with
+   `flake8` (E9, F63, F7, F82 plus bugbear `B` checks), `bandit`,
+   and `gitleaks` — install the CLI with
    `python -m pip install pre-commit`, then run
-   `pre-commit install` so they run on every commit; HTML/JS in
+   `pre-commit install` so they run on every commit. `mypy` on
+   `src/` is a manual hook while existing type errors are paid down
+   (`pre-commit run mypy --hook-stage manual`): please don't add new
+   errors in the files you touch. HTML/JS in
    `web_interface/` follows the patterns already in `templates/v3/`
    and `static/v3/`.
 5. **Update documentation** alongside code changes. If you add a