Detect mergeability regressions and auto-dispatch recovery work

[BASIC-BIT]

Parent epic: #43

Problem:

PRs can become unmergeable after unrelated changes land. VRDex wants the same recycler logic to recover mergeability issues automatically when practical.

Scope:

• define mergeability-regression detection as an event source
• define how the original implementer session should be resumed when practical
• define the expected recovery loop for merge conflicts or stale-branch issues

Non-goals:

• implementing perfect conflict resolution immediately
• guaranteeing zero human review for risky merges

Acceptance criteria:

• mergeability regression is captured as a first-class trigger
• resume-the-original-implementer is the default recovery policy when practical
• the behavior is documented clearly enough to become future automation work

Soft dependencies

• Define agentic review-recycle loop and trigger model #47 Define agentic review-recycle loop and trigger model
• Define orchestrator/supervisor loop and resumable-session policy #48 Define orchestrator/supervisor loop and resumable-session policy
• Define OpenCode task-pool/server direction for dispatched agent work #50 Define OpenCode task-pool/server direction for dispatched agent work

Soft dependents

• Define agentic review-recycle loop and trigger model #47 Define agentic review-recycle loop and trigger model