fix(agent): handle UnicodeDecodeError in _read_last_entry

history.jsonl may contain non-UTF-8 bytes (e.g. from email channel
binary content), causing auto compact to fail when reading the last
entry for summary generation. Catch UnicodeDecodeError alongside
FileNotFoundError and JSONDecodeError.

nanobot/agent/memory.py

@@ -290,7 +290,7 @@ class MemoryStore:
                 if not lines:
                     return None
                 return json.loads(lines[-1])
-        except (FileNotFoundError, json.JSONDecodeError):
+        except (FileNotFoundError, json.JSONDecodeError, UnicodeDecodeError):
             return None
 
     def _write_entries(self, entries: list[dict[str, Any]]) -> None: