refactor(memory): simplify read_unprocessed_history cursor guard

Replace verbose loop with one-liner list comprehension using
e.get("cursor", 0) to handle missing cursor keys.

nanobot/agent/memory.py

@@ -245,15 +245,7 @@ class MemoryStore:
 
     def read_unprocessed_history(self, since_cursor: int) -> list[dict[str, Any]]:
         """Return history entries with cursor > *since_cursor*."""
-        entries = []
-        for e in self._read_entries():
-            cursor = e.get("cursor")
-            if cursor is None:
-                logger.warning("Skipping history entry without cursor: {}", e.get("timestamp", "unknown"))
-                continue
-            if cursor > since_cursor:
-                entries.append(e)
-        return entries
+        return [e for e in self._read_entries() if e.get("cursor", 0) > since_cursor]
 
     def compact_history(self) -> None:
         """Drop oldest entries if the file exceeds *max_history_entries*."""