Support showing all saved conversations from /resume inside the TUI

Feature request

Please add a way to open all saved local conversations from inside the Codex TUI.

Current behavior

• codex resume --all works from the shell and shows saved sessions beyond the current working directory.
• Inside the TUI, /resume opens the saved conversation picker, but appears scoped to the current working directory.
• Typing /resume --all does not behave like the CLI flag. It treats --all as a search query and shows:

No saved chat found matching '--all'

Desired behavior

One of these would solve the issue:

1. Support /resume --all inside the TUI.
2. Add a TUI setting to make /resume show all local saved conversations by default.
3. Add a separate slash command, such as /resume-all.
4. Add a keymap action that opens the all-sessions resume picker.

Why this helps

Users often work across multiple repositories and directories. When already inside an active Codex TUI session, they may want to switch to another saved local conversation without exiting to the shell and running:

codex resume --all

This would make conversation switching more consistent between the CLI and TUI.