Keybindings: /agents dialog back navigation not rebindable (Settings context escape-only)

Summary

The /agents dialog's back/dismiss navigation cannot be rebound via ~/.claude/keybindings.json. Pressing a custom key (e.g. ctrl+]) does nothing — only escape works, and it appears hardcoded rather than exposed as a named action.

Context

The keybindings docs describe the Settings context as "escape-only dismiss", with no settings:cancel, settings:back, or equivalent in the available actions table.

Steps to reproduce

1. Add a custom cancel/back binding (e.g. ctrl+]) to all documented contexts in ~/.claude/keybindings.json
2. Null out escape in those same contexts
3. Open /agents and press ctrl+] to go back — nothing happens

Expected behavior

The back/dismiss action in /agents should be exposed as a named action (e.g. settings:dismiss) so it can be rebound like other actions.

Why this matters

Users running Claude Code inside Emacs need to remap escape and ctrl+g to avoid conflicts. Without a rebindable back action for /agents, navigation is broken after remapping.

Suggested fix

Expose a settings:dismiss named action for the Settings context and document it in the keybindings reference.