/compact: agent activity indicator fires immediately when next message is queued, making it look like the message was sent mid-compact

## Summary
When `/compact` is running and the user types and submits a new message, the message appears in BOTH the queued-message stack AND the main chat history, and the "working" activity indicator kicks on right away — as if the message had been sent to the LLM mid-compaction.

## Actual behavior (verified)
Functionally this is correct: the agent does not actually receive or respond to the queued message until `/compact` finishes. The bug is purely visual / UX.

## Expected
While `/compact` is still running, a newly typed message should:
- Stay visible only in the queued stack
- NOT appear duplicated in the main chat transcript
- NOT trigger the "working" / activity indicator

The activity indicator and transcript echo should only fire once `/compact` completes and the message is actually dispatched to the model.

## Why this matters
Users currently believe their message was sent prematurely and either re-send it, cancel, or assume `/compact` silently failed. The deceptive UI undermines trust in `/compact`.

Start at the /compact command flow and reproduce the issue by submitting a message during compaction. Trace when the queued-message stack, main transcript, and working indicator update. Done means the message stays only in the queue and the transcript and indicator update only after compaction dispatches it.

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