Allow LeRobot imports to publish into bucket-backed HFlow data roots

[kstonekuan]

Current behavior

hflow import lerobot parses --output-dir as a Path and describes it as a local destination at src/hflow/cli.py:297-302. The public importer also accepts output_dir: Path, keeps _lerobot_cache below it, and returns list[Path] at src/hflow/importers/lerobot.py:500-550.

The rest of HFlow treats a data root as local storage or s3://, gs://, and az:// through parse_storage_root() and the shared publish/fetch boundary at src/hflow/storage.py:630-655. A bucket-backed workspace currently needs an extra manual upload step after LeRobot import.

Definition of done

• The public importer and CLI accept every data-root form supported by parse_storage_root().
• Existing local-path behavior and CLI usage remain compatible.
• For a bucket destination, Hugging Face downloads, video slices, and MCAP construction use local staging. Source cache files aren't uploaded into the durable data root.
• Canonical episodes publish under landing/ through the existing StorageRoot boundary.
• prepared-manifest.json publishes after every selected episode object succeeds.
• The public result reports real remote object URIs for bucket outputs. It doesn't represent a remote object as a Path.
• A failed upload leaves no partial remote object or manifest claiming that the import completed.
• Missing bucket dependencies and provider authentication failures reach the CLI as concise errors with no traceback.
• Add behavioral coverage for both local and bucket-backed destinations using the existing storage test boundary.
• Update docs/how-to/import-lerobot-v3.md with one bucket example and explain where the local cache lives.

Batch retry and episode reuse are tracked separately in #303. The two designs should compose, but this issue doesn't add resumable imports itself.

Non-goals

• Airflow execution
• Parallel episode conversion
• Bucket or filesystem watchers
• A new storage backend or package
• Changes to the canonical MCAP layout

Validation

uv run ruff check
uv run ruff format --check
uv run ty check
uv run pytest tests/test_lerobot_converter.py tests/test_storage.py -q
uv run pytest -q