feat: keyboard modifier (and bindable command) to send the opposite of the default follow-up mode

[andybergon]

After #11673, steer-now is only reachable with the mouse (the arrow under the dashed bubble); Enter always uses the default. There is no keyboard way to do the other action for a single message.

Add a modifier such as Cmd/Ctrl+Enter that performs the opposite of the configured default (steer-now when the default is queue, queue-now when the default is steer), and expose it as a bindable keybinding command so it can be remapped.

For active threads Mod+Enter is currently unused (composer-logic.ts only uses it to pick foreground vs background for a new draft thread), so the chord is available. Complements #6817 (expose more bindable keybinding commands) and the default-mode setting in the companion request.

[01-1]

There is a keybind to queue instead of steer, but not steer instead of queue. Also it only acts on an already-queued message, it doesn't steer the current draft

[juliusmarminge]

Resolved by #12075: while a turn is running, Ctrl/⌘+Enter sends one follow-up with the opposite of the default Queue/Steer behavior (and Ctrl/⌘+Shift+Enter when the send shortcut already requires the modifier). Queue and Steer are also shown as explicit controls.