Migrate babysit-pr repository review remediation to review-fix-loop

[shaug]

Outcome

Migrate babysit-pr repository review/remediation to the packaged review-fix-loop skill while preserving PR monitoring and merge-authority boundaries.

Design: design/review-fix-loop.md
Related epic: #95

This is an intentionally separate fast follow, not a subissue of #95.

Scope

• Delegate only the repository review/fix loop.
• Map update_pr publication and retained-commit outcomes into the watcher state machine.
• Preserve CI monitoring, review-thread handling, merge readiness, and merge authority in babysit-pr.

Non-goals

• Moving PR lifecycle management into review-fix-loop.
• Migrating other callers.

Acceptance criteria

• Repository remediation uses the standalone skill without publishing intermediate fixes.
• PR-head changes and publication races produce safe watcher transitions.
• Non-converged outcomes surface retained unpushed commits prominently.
• Merge authority and lifecycle behavior remain unchanged.
• Existing watcher evaluations continue to pass, with new migration coverage.

Dependency

Blocked by the packaged-skill leaf of #95.