fix(paired): break reviewer step_done ⇄ owner task_done infinite loop

The reviewer step_done verdict mapped to request_owner_changes
unconditionally, bypassing the deadlock guard that every sibling
verdict respects. When the owner kept replying task_done and the
reviewer kept replying step_done, the room oscillated forever,
spamming duplicate reviewer messages into the channel.

Apply the same `roundTripCount >= deadlockThreshold` guard used by
the done_with_concerns/continue/default branch so step_done
oscillation escalates to the arbiter (or completes via escalation
when no arbiter) instead of looping. Reuses existing round_trip_count
and deadlockThreshold; no migration needed.

Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>

src/paired-execution-context-shared.test.ts

@@ -166,6 +166,26 @@ describe('paired execution context shared verdict helpers', () => {
     ).toEqual({ kind: 'request_arbiter' });
   });
 
+  it('routes reviewer step_done to arbiter once round trips hit the deadlock threshold', () => {
+    // Below threshold: keep iterating with the owner.
+    expect(
+      resolveReviewerCompletionSignal({
+        visibleVerdict: 'step_done',
+        roundTripCount: 1,
+        deadlockThreshold: 2,
+      }),
+    ).toEqual({ kind: 'request_owner_changes' });
+    // At/above threshold: break the owner⇄reviewer step_done oscillation by
+    // escalating to the arbiter instead of looping forever.
+    expect(
+      resolveReviewerCompletionSignal({
+        visibleVerdict: 'step_done',
+        roundTripCount: 2,
+        deadlockThreshold: 2,
+      }),
+    ).toEqual({ kind: 'request_arbiter' });
+  });
+
   it('maps reviewer failure verdicts to explicit failure signals', () => {
     expect(
       resolveReviewerFailureSignal({

src/paired-execution-context-shared.ts

@@ -112,6 +112,9 @@ export function resolveReviewerCompletionSignal(args: {
     case 'done':
       return { kind: 'request_owner_finalize' };
     case 'step_done':
+      if (roundTripCount >= deadlockThreshold) {
+        return { kind: 'request_arbiter' };
+      }
       return { kind: 'request_owner_changes' };
     case 'blocked':
     case 'needs_context':