fix(lerobot): namespace cache by resolved revision

[mikemikimike]

Summary

• Namespace LeRobot source caches by the resolved immutable repository SHA.
• Prevent reused output directories from mixing artifacts across revisions.
• Preserve same-SHA cache reuse and manifest provenance.

Closes #294

Validation

• ruff check, ruff format --check, ty check
• focused LeRobot converter tests
• full pytest suite: 1250 passed, 8 skipped

[github-actions]

👋 Hi @mikemikimike — thank you so much for your first contribution to HFlow!

A maintainer will review your pull request as soon as possible. In the meantime:

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💡 Tip: one open pull request per contributor at a time. Issues with an assignee are taken; everything else is fair game.

We are excited to have you here and appreciate your help making the project better! 🙌