bug(v2): compaction can fail to start with no actionable reason

[opencode-agent]

Observed

In the V2 TUI, a high-context session (270.3k) emitted two consecutive compaction failures after a build completed:

Compaction could not start

The UI did not surface the reason compaction was rejected or provide a recovery action. The session was using Little Frank / anomaly (OpenAI) at high reasoning effort.

Observed at 2026-07-09 00:57 UTC. Source Slack thread.

Expected

• A compaction request either starts or returns a typed, actionable reason it cannot start.
• Repeated triggers do not render duplicate opaque failures without a state transition.
• The session remains recoverable and the user can retry or continue safely.

Track

• Identify the state/precondition or race that rejects compaction startup.
• Carry a structured failure reason through the V2 event model and render it in the TUI.
• Add regression coverage for repeated compaction attempts at the high-context transition.