/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.

Written by the indexing model from the issue text.