Fix running background status in s_full

Keep the reference agent's background status output readable by reporting '(running)' instead of leaking a None placeholder for unfinished tasks.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

tests/test_s_full_background.py

@@ -0,0 +1,67 @@
+import importlib.util
+import os
+import sys
+import tempfile
+import types
+import unittest
+from pathlib import Path
+
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+MODULE_PATH = REPO_ROOT / "agents" / "s_full.py"
+
+
+def load_s_full_module(temp_cwd: Path):
+    fake_anthropic = types.ModuleType("anthropic")
+
+    class FakeAnthropic:
+        def __init__(self, *args, **kwargs):
+            self.messages = types.SimpleNamespace(create=None)
+
+    fake_dotenv = types.ModuleType("dotenv")
+    setattr(fake_anthropic, "Anthropic", FakeAnthropic)
+    setattr(fake_dotenv, "load_dotenv", lambda override=True: None)
+
+    previous_anthropic = sys.modules.get("anthropic")
+    previous_dotenv = sys.modules.get("dotenv")
+    previous_cwd = Path.cwd()
+    spec = importlib.util.spec_from_file_location("s_full_under_test", MODULE_PATH)
+    if spec is None or spec.loader is None:
+        raise RuntimeError(f"Unable to load {MODULE_PATH}")
+    module = importlib.util.module_from_spec(spec)
+
+    sys.modules["anthropic"] = fake_anthropic
+    sys.modules["dotenv"] = fake_dotenv
+    try:
+        os.chdir(temp_cwd)
+        os.environ.setdefault("MODEL_ID", "test-model")
+        spec.loader.exec_module(module)
+        return module
+    finally:
+        os.chdir(previous_cwd)
+        if previous_anthropic is None:
+            sys.modules.pop("anthropic", None)
+        else:
+            sys.modules["anthropic"] = previous_anthropic
+        if previous_dotenv is None:
+            sys.modules.pop("dotenv", None)
+        else:
+            sys.modules["dotenv"] = previous_dotenv
+
+
+class BackgroundManagerTests(unittest.TestCase):
+    def test_check_returns_running_placeholder_when_result_is_none(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            module = load_s_full_module(Path(tmp))
+            manager = module.BackgroundManager()
+            manager.tasks["abc123"] = {
+                "status": "running",
+                "command": "sleep 1",
+                "result": None,
+            }
+
+            self.assertEqual(manager.check("abc123"), "[running] (running)")
+
+
+if __name__ == "__main__":
+    unittest.main()