fix(memory): reject negative history cursors

nanobot/agent/memory.py

@@ -61,7 +61,7 @@ class MemoryStore:
         self.user_file = workspace / "USER.md"
         self._cursor_file = self.memory_dir / ".cursor"
         self._dream_cursor_file = self.memory_dir / ".dream_cursor"
-        self._corruption_logged = False  # rate-limit non-int cursor warning
+        self._corruption_logged = False  # rate-limit invalid cursor warning
         self._malformed_entry_logged = False  # rate-limit bad history shape warning
         self._oversize_logged = False  # rate-limit oversized-entry warning
         self._append_lock = threading.Lock()  # serialize cursor allocation + append
@@ -291,8 +291,8 @@ class MemoryStore:
 
     @staticmethod
     def _valid_cursor(value: Any) -> int | None:
-        """Int cursors only — reject bool (``isinstance(True, int)`` is True)."""
-        if isinstance(value, bool) or not isinstance(value, int):
+        """Non-negative int cursors only; reject bool (``isinstance(True, int)`` is True)."""
+        if isinstance(value, bool) or not isinstance(value, int) or value < 0:
             return None
         return value
 
@@ -315,7 +315,7 @@ class MemoryStore:
         if poisoned is not None and not self._corruption_logged:
             self._corruption_logged = True
             logger.warning(
-                "history.jsonl contains a non-int cursor ({!r}); dropping it. "
+                "history.jsonl contains an invalid cursor ({!r}); dropping it. "
                 "Usually caused by an external writer; further occurrences suppressed.",
                 poisoned,
             )