chore(CodeReviewQueue): default missing repo to '' in push envelope

Defensive null-coalesce in buildPushEnvelope() matching the sibling fields
(ref/before_sha/after_sha already use ?? ''). Picked up as an unrelated drive-by
edit from a worker-refactor agent; harmless and consistent, kept on its own.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: detain

src/CodeReviewQueue.php

@@ -276,7 +276,7 @@ public static function buildPushEnvelope(array $job): array
             'id' => self::uuidV4(),
             'ts' => time(),
             'kind' => 'push',
-            'repo' => $job['repo'],
+            'repo' => $job['repo'] ?? '',
             'action' => 'push',
             'ref' => $job['ref'] ?? '',
             'before_sha' => $job['before_sha'] ?? '',