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.ts

@@ -846,6 +846,7 @@ export function createMessageRuntime(deps: MessageRuntimeDeps): {
               if (task) {
                 updatePairedTask(task.id, {
                   status: 'completed',
+                  completion_reason: 'stopped',
                   updated_at: new Date().toISOString(),
                 });
               }