test: blocked-check 用 _req_safe 替代 _req 避免 API 错误误判 - Closes #58

- 新增 _req_safe()：API 错误返回 None 而非 sys.exit(1)
- blocked_check / _unblock_issues_blocked_by / _get_blocking_refs 改用 _req_safe
- API 失败时保守处理：保持 blocked 状态

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/agent_poller.py

@@ -124,12 +124,7 @@ def blocked_check():
     If no references found or all referenced issues are closed,
     removes the 'blocked' label.
     """
-    try:
-        all_blocked = _req("GET", "/issues?state=open&labels=blocked")
-    except SystemExit:
-        print("No blocked issues found.")
-        return
-
+    all_blocked = _req_safe("GET", "/issues?state=open&labels=blocked")
     if not all_blocked:
         print("No blocked issues found.")
         return