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-agent-executor.test.ts

@@ -361,6 +361,7 @@ describe('runAgentForGroup room memory', () => {
         status: 'active',
         arbiter_verdict: null,
         arbiter_requested_at: null,
+        completion_reason: null,
         created_at: '2026-03-29T00:00:00.000Z',
         updated_at: '2026-03-29T00:00:00.000Z',
       },
@@ -425,6 +426,7 @@ describe('runAgentForGroup room memory', () => {
         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',
       },
@@ -514,6 +516,7 @@ describe('runAgentForGroup room memory', () => {
         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',
       },