Today there's no way to remove a queued message — ↑ loads it for edit,
ctrl-K dispatches the head, but a draft you no longer want stays put
forever. ctrl-C just clears the composer and exits edit mode without
touching the queue.
Two new bindings, both gated on queueEditIdx !== null so they're
inert when the user isn't pointing at a queue item:
- ctrl-X — delete the queue item being edited, clear composer, exit
edit mode. "cut" matches the mental model and doesn't collide with
any existing binding.
- esc — cancel the edit (composer clears, item stays in queue).
Mirrors ctrl-C's existing behavior so muscle memory has two paths.
Header line now reads `queued (3) · editing 2 · ⌃X delete · esc cancel`
when in edit mode, so the affordance is discoverable without /help.
The /help hotkey table also gets a Ctrl+X entry.
ctrl-C is intentionally unchanged: it should never destroy queued
content. Cancel is non-destructive (esc / ctrl-C); only ctrl-X
removes the item.
