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/types.ts

@@ -71,6 +71,7 @@ export interface PairedTask {
   status: PairedTaskStatus;
   arbiter_verdict: string | null;
   arbiter_requested_at: string | null;
+  completion_reason: string | null;
   created_at: string;
   updated_at: string;
 }