feat: support uv.lock as version-file - #379

Adds uv.lock as a supported version-file format, alongside the existing
pyproject.toml and requirements.txt. A uv.lock pins the exact resolved
ruff version, so reading it gives a reproducible install that matches the
project's lockfile — the single source of truth — instead of the lower-bound /
range typically declared in pyproject.toml.

The parser reads the [[package]] entry whose name is ruff and returns its
version. If ruff is absent or the file cannot be parsed, the action warns and
falls back to latest (same behaviour as the other formats).

Validation:

• npm run all — tsc typecheck clean, biome clean, dist rebuilt, jest 65/65 pass
• Added unit tests (__tests__/version/file-parser.test.ts) + a uv.lock fixture
• Added an integration job (test-default-version-from-uv-lock) that runs the
  action against the fixture and asserts ruff 0.9.5 is installed

closes #375