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/run.py

@@ -990,7 +990,7 @@ class GatewayRunner:
         # Memory flush before reset: load the old transcript and let a
         # temporary agent save memories before the session is wiped.
         try:
-            old_entry = self.session_store._sessions.get(session_key)
+            old_entry = self.session_store._entries.get(session_key)
             if old_entry:
                 old_history = self.session_store.load_transcript(old_entry.session_id)
                 if old_history:
@@ -1222,9 +1222,9 @@ class GatewayRunner:
         if not last_user_msg:
             return "No previous message to retry."
         
-        # Truncate history to before the last user message
+        # Truncate history to before the last user message and persist
         truncated = history[:last_user_idx]
-        session_entry.conversation_history = truncated
+        self.session_store.rewrite_transcript(session_entry.session_id, truncated)
         
         # Re-send by creating a fake text event with the old message
         retry_event = MessageEvent(
@@ -1256,7 +1256,7 @@ class GatewayRunner:
         
         removed_msg = history[last_user_idx].get("content", "")
         removed_count = len(history) - last_user_idx
-        session_entry.conversation_history = history[:last_user_idx]
+        self.session_store.rewrite_transcript(session_entry.session_id, history[:last_user_idx])
         
         preview = removed_msg[:40] + "..." if len(removed_msg) > 40 else removed_msg
         return f"↩️ Undid {removed_count} message(s).\nRemoved: \"{preview}\""
@@ -1330,7 +1330,7 @@ class GatewayRunner:
                 lambda: tmp_agent._compress_context(msgs, "", approx_tokens=approx_tokens),
             )
 
-            session_entry.conversation_history = compressed
+            self.session_store.rewrite_transcript(session_entry.session_id, compressed)
             new_count = len(compressed)
             new_tokens = estimate_messages_tokens_rough(compressed)