fix: advance dream cursor when Dream is disabled to prevent prompt bloat (#4242)

When dream.enabled is false, the Dream cron job never runs, so the
dream cursor (.dream_cursor) stays at its initial value (0). This
causes read_recent_history_for_prompt() to treat every history entry
as unprocessed, injecting the full chat history into every system
prompt and growing without bound.

Fix: fast-forward the dream cursor to the latest history entry at
gateway startup when Dream is disabled.

nanobot/agent/memory.py

@@ -479,6 +479,9 @@ class MemoryStore:
     def set_last_dream_cursor(self, cursor: int) -> None:
         self._dream_cursor_file.write_text(str(cursor), encoding="utf-8")
 
+    def get_latest_cursor(self) -> int:
+        return max(self._next_cursor() - 1, 0)
+
     def build_dream_prompt(self, *, max_entries: int = 20) -> tuple[str, int] | None:
         """Build the Dream prompt with unprocessed history context.