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/db.test.ts

@@ -561,6 +561,7 @@ describe('paired task state', () => {
       status: 'active',
       arbiter_verdict: null,
       arbiter_requested_at: null,
+      completion_reason: null,
       created_at: '2026-03-28T00:00:00.000Z',
       updated_at: '2026-03-28T00:00:00.000Z',
     });
@@ -602,6 +603,7 @@ describe('paired task state', () => {
       status: 'active',
       arbiter_verdict: null,
       arbiter_requested_at: null,
+      completion_reason: null,
       created_at: '2026-03-28T00:00:00.000Z',
       updated_at: '2026-03-28T00:00:00.000Z',
     });