[FEATURE] /undo during active generation: cancel the queued messages, keep the in-flight stream

Summary

Proposed semantics for /undo while a generation is active: cancel the queued messages and leave the in-flight stream alone, applying the revert once the active response completes.

Today the behavior is either a Session is busy failure (#38476) or — in the pre-beta behavior #38476 asks to restore — aborting the active generation. Both miss the common case.

The common case

A run is active and I realize the follow-up message I just queued is wrong (typo, wrong file, changed my mind). I reach for /undo. What I want reverted is the queued message — the part that has not been consumed yet. What I do not want is to kill the active generation:

• On paid providers, aborting mid-stream throws away committed tokens.
• On large local models (e.g. long-context local seats), prefill + decode can represent minutes of compute that must then be redone from scratch.

Esc/ctrl+c already exists as the explicit "kill the active generation" affordance. /undo does not need to be a second one.

Proposed behavior

This also gives a natural answer to the promotion race in #39736: a queued-but-not-yet-promoted message becomes an explicit "cancelled before promotion" outcome rather than an undo boundary the revert planner cannot find.

Why not simply restore abort+undo (the ask in #38476)

It works, but it conflates two operations and pays for both: the active generation is destroyed and the message is reverted. With queue-first semantics, #38476's reproduction (start response → /undo → error) resolves without any abort when the intent was fixing the queue, and Esc remains available when the intent really is killing the run.

Environment

• opencode 1.18.15, macOS TUI (arm64), Homebrew
• Abort path observed in local logs as message=cancel followed by message=process ... error=Aborted

Refs: #38476 (busy failure; asks for abort+undo), #39736 (admitted-message undo race after interrupt), #14014 (historical: session.undo aborted ongoing work)