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

hermes_state.py

@@ -476,6 +476,17 @@ class SessionDB:
             results.append({**session, "messages": messages})
         return results
 
+    def clear_messages(self, session_id: str) -> None:
+        """Delete all messages for a session and reset its counters."""
+        self._conn.execute(
+            "DELETE FROM messages WHERE session_id = ?", (session_id,)
+        )
+        self._conn.execute(
+            "UPDATE sessions SET message_count = 0, tool_call_count = 0 WHERE id = ?",
+            (session_id,),
+        )
+        self._conn.commit()
+
     def delete_session(self, session_id: str) -> bool:
         """Delete a session and all its messages. Returns True if found."""
         cursor = self._conn.execute(