Unsent messages to remote sessions can now be taken back

What

• Remote/cloud sessions now track when a queued message is "held" (queued but not yet sent to the cloud session) and could still be withdrawn. RemoteSessionManager calls a new tellWithholdableHeld() method on release, gate resolution, withdrawal, and settlement, and fires an onWithholdableHeldChanged callback when that status changes; the UI tracks this as a cloudMessageHeld state.
• Users can now press Escape while idle to reclaim messages that were queued but not yet sent, via a new cancelHeldMessages capability, instead of triggering a full interrupt. This shows a notice and logs telemetry.
• Cancelling with an empty prompt while in remote mode now also cancels held remote messages, via the same cancelHeldMessages path.
• Taking back the very first prompt (the one given on the command line, which starts out withheld) now shows a distinct notice: "You took back your first message (the one given on the command line) — it wasn't sent to the cloud session."

Why

Previously a message queued for a remote/cloud session but not yet sent couldn't easily be pulled back. Now pressing Escape, or cancelling with an empty prompt, retracts it before it reaches the cloud session, with clear feedback about what happened.