Support nested task dataset layouts when pushing to Hub

[karatarassul4-max]

Fixes #96

Summary

• reuse _list_task_dirs() in push_to_hub() so Hub staging and image preparation share the same discovery logic
• support both flat task layouts and the canonical tasks/<id>/task.toml layout
• keep hidden directories excluded and preserve deterministic ordering
• reject duplicate task IDs across flat and nested layouts instead of silently choosing one copy
• add regression coverage for nested discovery, mixed layouts, duplicate IDs, and nested push_to_hub() metadata discovery

Validation

• uv run pytest -q tests/registry/test_integration.py tests/test_hub_metadata_read.py → 34 passed
• uv run ruff check src/repo2rlenv/registry/integration.py src/repo2rlenv/hub.py tests/registry/test_integration.py tests/test_hub_metadata_read.py → passed
• uv run ruff format --check src/repo2rlenv/registry/integration.py src/repo2rlenv/hub.py tests/registry/test_integration.py tests/test_hub_metadata_read.py → 4 files already formatted
• git diff --check → clean
• full uv run pytest -q on Windows: 712 passed, 6 skipped, 7 platform-specific failures related to executable bits, path separators, and Windows local-path parsing; none are in files changed by this PR

[KNambiarDJsc]

I reviewed and tested 92b5138:

• CI-equivalent run on Linux: ruff is clean, and the full suite passes on py3.12 and py3.14 (719 passed).
• End-to-end on a real nested dataset. I copied three tasks from the published AdithyaSK/repo2rlenv-pr-diff dataset in its tasks/<id>/ layout. I also added a root README.md and registry.json, as huggingface-cli download leaves them. Then I ran push_to_hub with HfApi mocked.
  • On main it raises RuntimeError: no Harbor tasks found.
  • With this PR, all 3 tasks are staged under tasks/, and the generated manifest.json has the same 3 rows.
  • Nothing ends up double-nested as tasks/tasks/.
  • The push runs through image preparation without errors; these tasks ship environment/Dockerfiles.

LGTM.

One non-blocking nit: hub.py now imports the private _list_task_dirs from registry.integration. If both modules are meant to share discovery, dropping the underscore would make that dependency explicit.

[adithya-s-k]

thanks @karatarassul4-max, merged! appreciate the nested task layout support and the duplicate ID checks.