fix: /retry, /undo, /compress, and /reset gateway commands (#210)

- /retry, /undo, /compress were setting a non-existent conversation_history
  attribute on SessionEntry (a @dataclass with no such field). The dangling
  attribute was silently created but never read — transcript was reloaded
  from DB on next interaction, making all three commands no-ops.

- /reset accessed self.session_store._sessions (non-existent) instead of
  self.session_store._entries, causing AttributeError caught by a bare
  except, silently skipping the pre-reset memory flush.

Fix:
- Add SessionDB.clear_messages() to delete messages and reset counters
- Add SessionStore.rewrite_transcript() to atomically replace transcript
  in both SQLite and legacy JSONL storage
- Replace all dangling attr assignments with rewrite_transcript() calls
- Fix _sessions → _entries in /reset handler

Closes #210

gateway/session.py

@@ -567,6 +567,34 @@ class SessionStore:
         with open(transcript_path, "a") as f:
             f.write(json.dumps(message, ensure_ascii=False) + "\n")
     
+    def rewrite_transcript(self, session_id: str, messages: List[Dict[str, Any]]) -> None:
+        """Replace the entire transcript for a session with new messages.
+        
+        Used by /retry, /undo, and /compress to persist modified conversation history.
+        Rewrites both SQLite and legacy JSONL storage.
+        """
+        # SQLite: clear old messages and re-insert
+        if self._db:
+            try:
+                self._db.clear_messages(session_id)
+                for msg in messages:
+                    self._db.append_message(
+                        session_id=session_id,
+                        role=msg.get("role", "unknown"),
+                        content=msg.get("content"),
+                        tool_name=msg.get("tool_name"),
+                        tool_calls=msg.get("tool_calls"),
+                        tool_call_id=msg.get("tool_call_id"),
+                    )
+            except Exception as e:
+                logger.debug("Failed to rewrite transcript in DB: %s", e)
+        
+        # JSONL: overwrite the file
+        transcript_path = self.get_transcript_path(session_id)
+        with open(transcript_path, "w") as f:
+            for msg in messages:
+                f.write(json.dumps(msg, ensure_ascii=False) + "\n")
+
     def load_transcript(self, session_id: str) -> List[Dict[str, Any]]:
         """Load all messages from a session's transcript."""
         # Try SQLite first