Addresses #12178
Problem: The TUI /rename prompt opened blank even when the current
thread already had a custom name, making small edits awkward.
Solution: Let custom prompts receive initial text and prefill /rename
with the existing thread name while preserving the empty prompt for
unnamed threads.
Testing: Manually verified that the feature works by using `/rename`
with unnamed and already-named threads.
Eric Traut
9c56e89e4f