fix: return "deny" on approval callback timeout instead of None

_approval_callback() had no return statement after the timeout break,
causing it to return None. Callers expect a string ("once", "session",
"always", or "deny"), so None could lead to undefined behavior when
approving dangerous commands.

cli.py

@@ -2245,6 +2245,8 @@ class HermesCLI:
         self._approval_state = None
         self._approval_deadline = 0
         self._invalidate()
+        return "deny"
+
     def chat(self, message, images: list = None) -> Optional[str]:
         """
         Send a message to the agent and get a response.