CI hygiene: drop dead env, bump actions, cache pip, enforce ruff

[SaInekK]

Five small CI/import fixes, one commit each:

1. Drop HARMONY_LITE=no_transformers from test.yml and docker.yml. Per CLAUDE.md, this env var is set but never read in code — the real switches are HARMONY_NO_PARSING / HARMONY_NO_MATCHING. CI was running the full stack regardless of this var; dropping it removes a misleading signal without changing what CI exercises.
2. Bump actions/checkout@v2 → v4 and actions/setup-python@v2 → v5 in test.yml. docker.yml already uses v4. v2 emits deprecation warnings on each run.
3. Add cache: 'pip' with cache-dependency-path: pyproject.toml to the setup-python step. Torch and transformers are heavy wheels — caching saves significant time per run.
4. Enforce ruff lint in CI. PR add ruff config and clean unused imports #130 added the config ([tool.ruff] in pyproject.toml, pyflakes-only) but no step ran it. Adding ruff check src tests before tox ensures the config stays useful.
5. Replace print() with warnings.warn(..., ImportWarning) in src/harmony/__init__.py for the "transformers not available" path. Library code shouldn't write to stdout; ImportWarning is the standard signal.

All changes are mechanical and zero-risk to test behaviour. Verified locally that import harmony still works (both full and HARMONY_NO_MATCHING=1 modes), and ruff check src tests passes against the current config.