Clarify worktree hygiene warnings

[yvette-carlisle]

Summary

• Add structured warning_details entries for operator snapshot warnings.
• Explain worktree_hygiene_unavailable with project, repo path, failure reason, and next action.
• Render the clearer warning in status text and the operator dashboard, with regression coverage.

Verification

• cargo make fmt
• cargo make lint-fix
• cargo make test (916 passed, 1 skipped)
• git diff --check