V2 Slice 16: Required Provider Check failure repairs and merges

[notadamking]

Source PRD: #64

What to build

Prove self-healing for a required Provider Check failure: GitHub check fails, Repair Context is attached, provider Worker repair Assignment updates the same PR, checks/review/policy rerun, and merge completes.

Acceptance criteria

• A terminal failing required Provider Check creates a Repair Trigger and stable Repair Context item.
• Repair Context includes check name, head SHA, provider URL, bounded failure context, and provenance.
• Scheduler dispatches a new task-owned repair Assignment and provider Worker Session.
• Repair updates the same PR branch and resolves or supersedes targeted Repair Context.
• Verification, review, policy, and merge continue after repair without losing lineage.

Blocked by

• V2 Slice 12: Autopilot Agent Review merges without Human Review #76