rename feature implementation

[jevon]

Summary

• add rename mode (Ctrl-R) so the selector can change an existing try’s basename without leaving the TUI, based this on the tests that already existed but were failing
• show rename dialog with the 📝 header, current/new name fields, inline cursor, and validation (empty/slash/existing names)
• update spec/tests/test_31_rename.sh so its footer hint matches the Ctrl-R convention that the UI already uses
• emit the rename script through (cd → mv → cd) so the shell wrapper can apply the rename and land in the new directory

Testing

• ./spec/tests/test_31_rename.sh now passes w/ modification for UI hint

Notes

simple case changes cause a collision in any filesys that is case insensitive. didn't add checking for that.