feat: add completion_reason to paired_tasks (done/escalated/stopped/arbiter_escalated)

- owner can summon arbiter via BLOCKED/NEEDS_CONTEXT verdict
- completion_reason tracks why task ended: done (success), escalated
  (reviewer gave up), stopped (/stop), arbiter_escalated

src/message-runtime.test.ts

@@ -565,6 +565,7 @@ describe('createMessageRuntime', () => {
       status: 'review_ready',
       arbiter_verdict: null,
       arbiter_requested_at: null,
+      completion_reason: null,
       created_at: '2026-03-30T00:00:00.000Z',
       updated_at: '2026-03-30T00:00:00.000Z',
     });