refine heartbeat session retention boundaries

nanobot/session/manager.py

@@ -98,6 +98,32 @@ class Session:
         self.last_consolidated = 0
         self.updated_at = datetime.now()
 
+    def retain_recent_legal_suffix(self, max_messages: int) -> None:
+        """Keep a legal recent suffix, mirroring get_history boundary rules."""
+        if max_messages <= 0:
+            self.clear()
+            return
+        if len(self.messages) <= max_messages:
+            return
+
+        start_idx = max(0, len(self.messages) - max_messages)
+
+        # If the cutoff lands mid-turn, extend backward to the nearest user turn.
+        while start_idx > 0 and self.messages[start_idx].get("role") != "user":
+            start_idx -= 1
+
+        retained = self.messages[start_idx:]
+
+        # Mirror get_history(): avoid persisting orphan tool results at the front.
+        start = self._find_legal_start(retained)
+        if start:
+            retained = retained[start:]
+
+        dropped = len(self.messages) - len(retained)
+        self.messages = retained
+        self.last_consolidated = max(0, self.last_consolidated - dropped)
+        self.updated_at = datetime.now()
+
 
 class SessionManager:
     """