fix(security): restore type safety and extract constant in shell hook block handler

Address code review feedback on _parse_response:

1. Restore isinstance(raw, str) guard so non-string message/reason values
   (e.g. integers, lists) from a malformed hook response fall back to the
   default rather than being forwarded as-is. This keeps the contract that
   message in the returned dict is always a string.

2. Extract the repeated literal 'Blocked by shell hook.' into a module-level
   constant _DEFAULT_BLOCK_MESSAGE to avoid duplication and make it easy to
   change in one place.

Four new unit tests added to tests/agent/test_shell_hooks.py covering:
- action block with no message (uses default)
- decision block with no reason (uses default)
- action block with empty string message (uses default)
- action block with non-string message, e.g. integer (uses default)

agent/shell_hooks.py

@@ -83,6 +83,7 @@ logger = logging.getLogger(__name__)
 DEFAULT_TIMEOUT_SECONDS = 60
 MAX_TIMEOUT_SECONDS = 300
 ALLOWLIST_FILENAME = "shell-hooks-allowlist.json"
+_DEFAULT_BLOCK_MESSAGE = "Blocked by shell hook."
 
 # (event, matcher, command) triples that have been wired to the plugin
 # manager in the current process.  Matcher is part of the key because
@@ -515,10 +516,12 @@ def _parse_response(event: str, stdout: str) -> Optional[Dict[str, Any]]:
 
     if event == "pre_tool_call":
         if data.get("action") == "block":
-            message = data.get("message") or data.get("reason") or "Blocked by shell hook."
+            raw = data.get("message") or data.get("reason")
+            message = raw if isinstance(raw, str) and raw else _DEFAULT_BLOCK_MESSAGE
             return {"action": "block", "message": message}
         if data.get("decision") == "block":
-            message = data.get("reason") or data.get("message") or "Blocked by shell hook."
+            raw = data.get("reason") or data.get("message")
+            message = raw if isinstance(raw, str) and raw else _DEFAULT_BLOCK_MESSAGE
             return {"action": "block", "message": message}
         return None