[Feature Request] Add submit modifier to queue messages as follow-ups instead of steering

Bug Description
Title: Add a submit modifier to queue a message as a follow-up instead of steering

Body:

When I submit a message while the agent is still running, it's treated as a steering message — injected after the current in-flight tool call to redirect the active turn. Sometimes
that's what I want, but often I want the opposite: queue my message as a follow-up that's only processed after the current turn fully completes, without interrupting or redirecting the
in-progress work.

Today there's no way to choose. All mid-run input defaults to steering — no keybinding, no keybindings.json action, no settings.json flag.

Request: A way to opt into follow-up/queued semantics at submit time. Ideal options:

1. A submit modifier (e.g. Alt+Enter / Ctrl+Enter) that queues-as-follow-up instead of steering.
2. A bindable keybindings.json action (e.g. chat:submitQueued) so the choice is configurable.
3. Optionally a settings.json toggle to set the default behavior for mid-run submits.

Steering and follow-up are genuinely different intents; right now only one is reachable.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.186
• Feedback ID: 4db44634-46f9-4366-9db1-08f340e71e67

Errors

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