Docs: CONTRIBUTING.md lists two opt-in test suites; there are three

CONTRIBUTING.md says two integration test suites are opt-in. There are three. ## Current behavior `CONTRIBUTING.md:66-72` says "Two integration tests are intentionally opt-in because they need network or Docker access" and lists: ```bash HFLOW_NETWORK_TESTS=1 uv run pytest tests/test_ffmpeg.py -q HFLOW_DOCKER_TESTS=1 uv run pytest tests/test_runtime_integration.py -q ``` A third gate exists: `tests/test_storage.py:333` skips `TestLiveBucket` unless `HFLOW_TEST_BUCKET_URL` is set (`reason="set HFLOW_TEST_BUCKET_URL=gs://bucket/tmp-prefix to run live"`). ## What to do Change "Two" to "Three", adjust the reason clause (network, Docker, or a live bucket), and add the third command line: ```bash HFLOW_TEST_BUCKET_URL=gs://your-bucket/tmp-prefix uv run pytest tests/test_storage.py -q ``` Scope note: this is a test-only gate documented in CONTRIBUTING.md. It is separate from #9, which asks for a runtime `HFLOW_*` reference page; do not merge the two. ## Definition of done - CONTRIBUTING.md lists all three opt-in suites with runnable commands. ## Validation ```bash lychee --no-progress --include-fragments \ --exclude '^https://github\.com/Hebbian-Robotics/hflow/(issues|security/advisories/new)$' \ --exclude-path references/mcap-spec.md \ --exclude-path references/foxglove-CompressedVideo.proto . ```