[FEATURE] claude agents: confirm delay before Ctrl+X deletes a session/group

Problem

In claude agents, Ctrl+X arms a delete ("ctrl+x again to delete") and a second Ctrl+X deletes the selected session — or every session in a selected group. A quick double-press, or a stray key repeat, deletes immediately, so it's easy to remove the wrong session or a whole group by accident.

The delete key is hard-coded: it isn't an agents:* action in keybindings.json, and the plugin/mod API has no hook into the agents view, so users can't add their own guard.

Proposal

Add a short confirm delay to the agents-view delete:

1. First Ctrl+X shows a yellow countdown (e.g. 2s) instead of the red warning.
2. Ctrl+X presses during the countdown are ignored.
3. When the countdown ends, the existing red "ctrl+x again to delete" warning appears.
4. Ctrl+X after that deletes the session/group, as today.
5. Any other key (arrows, Esc) cancels, as today.

Nice to have:

• A setting for the delay (0 keeps today's behaviour).
• Expose delete as a rebindable keybinding action (e.g. agents:delete) so it can be moved off Ctrl+X.

Environment

Claude Code 2.1.294, macOS, cmux terminal.