[FEATURE] Prefill the rename input with the current session name

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Renaming a session in the CLI means typing the whole name again. I usually just want to tweak the existing name, for example fix a typo or add a suffix. /rename with no argument creates a brand-new name, and there's no way to bring back the current one.

Proposed Solution

When renaming interactively (/rename with no argument as an option, and Ctrl+R in the session picker and agents view), open the input with the current session name already filled in and the cursor at the end, so it can be edited in place.

Alternative Solutions

Tab-completing /rename to the current name would also work.

Priority

Low - Nice to have

Feature Category

Interactive mode (TUI)