Clarify resume hints for renamed threads

What variant of Codex are you using?

CLI / TUI

What feature would you like to see?

When a thread is renamed, this message gets displayed:
"Thread renamed to {thread-name}, to resume this thread run codex resume {thread-name}"
Which does not work. In fact, duplicates among threads' names are allowed, so we cannot point a specific thread with its name. We have to give the thread id instead.

Also, regardless of renaming a thread, exiting the thread displays:
"To continue this session, run codex resume {thread-id}"
Which, I think, looks awkward.

I think we would need to polish a bit so:

1. /rename displays correct guide (unless we change the behaviour).
2. /exit also mentions the thread name if it's renamed.

For example, /rename may display
"Thread renamed to {thread-name}. To resume this thread run codex resume, then select {thread-name}({thread-id})"

/exit may display
"To continue this session, run codex resume, then select {thread-name} ({thread-id})".

Additional information

<img width="891" height="96" alt="Image" src="https://github.com/user-attachments/assets/3e2009c1-9d63-4fd5-9f73-29029e4bb1a9" />

A screenshot for Codex guiding to run codex resume {thread-name}, which is tried and failed.