fix: skip opencode undo when message has zero output

Prevents error when undoing messages that produced no output.
Checks for reply, partialOutput, or done status before sending
/undo command to opencode.

chat/server.js

@@ -17334,6 +17334,10 @@ async function handleUndoMessage(req, res, sessionId, messageId, userId) {
   }
 
   try {
+    // Check if message had any output - if not, skip the opencode undo
+    const hasOutput = !!(message.reply || message.partialOutput) || message.status === 'done';
+
+    if (hasOutput) {
       log('Sending undo command to opencode', { sessionId, messageId, opencodeSessionId: session.opencodeSessionId });
 
       const cliCommand = resolveCliCommand('opencode');
@@ -17348,6 +17352,9 @@ async function handleUndoMessage(req, res, sessionId, messageId, userId) {
         cwd: session.workspaceDir || REPO_ROOT,
         timeout: 30000
       });
+    } else {
+      log('Skipping undo command - message had zero output', { sessionId, messageId });
+    }
 
     // Clear todos from the message when undone
     if (message.todos) {