Defaulting to original name in session renaming

I have the implementation on a branch if it would be helpful: https://github.com/PremPrakashCodes/codex/tree/fix/rename-prompt-prefill

• Added a with_initial_text(text: &str) -> Self builder method to CustomPromptView in custom_prompt_view.rs. It calls the existing set_text_clearing_elements API on the

internal TextArea and then sets the cursor to text.len(), so the prompt opens with the caret at the end of the pre-filled text rather than at position 0.

• Updated show_rename_prompt() in chatwidget.rs to read the existing thread_name as Option<&str> via as_deref().filter(|n| !n.is_empty()), avoiding an extra String

allocation. When the thread already has a non-empty name, that borrowed slice is passed into the prompt via with_initial_text; otherwise the prompt opens blank as before.

• The prompt title is still chosen from the same condition: "Rename thread" when a name exists, "Name thread" when it doesn't.
• No change to the "Name thread" flow — unnamed threads still open with an empty field and the existing title.
• No change to the submit path — the entered name still flows through normalize_thread_name, so trimming and empty-name rejection behave exactly as before.
• No new dependencies; the with_initial_text builder follows the same consuming-self builder pattern already used elsewhere in CustomPromptView.

This will be included in the next release.