fix(tui): allow Enter to submit when no autocomplete matches - #11970

Fixed a bug where typing paths starting with / (like /home/user/file.txt read this file) in the TUI input would show "No matching items" and block the Enter key from submitting. fix #11965

Root cause: The return key handler in autocomplete.tsx unconditionally called select() and e.preventDefault(), even when there were no matching options. This prevented form submission.

Fix: Only intercept Enter when options().length > 0. When no commands match (user is typing a path), close the autocomplete panel and let the normal submit handler process the input without preventing default behavior.

Tested by inputting /Users/hiro/opencode/README.md tell me what the file says in TUI:

• Can type the full path
• Shows "No matching items" when no commands match
• Pressing Enter submits the input successfully
• Command autocomplete still works for actual commands like /models