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>

.gitignore

@@ -3,15 +3,13 @@ __pycache__/
 *.py[cod]
 *$py.class
 
-# Secrets
-config/config_secrets.json
-# Atomic writes leave these behind when a save or a test is interrupted;
-# the suite drops several per run.
-config/.config_secrets.json.tmp.*
-config/config.json
-config/config.json.backup
-config/wifi_config.json
-config/uninstalled_plugins.json
+# Secrets and per-device state. Everything the software writes into config/
+# is local to one device -- config.json, config_secrets.json, wifi_config.json,
+# ytm_auth.json (a login session), saved_repositories.json, font_overrides.json,
+# and the temp files atomic writes leave behind when interrupted -- so only
+# the templates are tracked. Listing files one by one missed several.
+config/*
+!config/*.template.json
 credentials.json
 token.pickle