/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`.

從 /compact 命令流程開始，透過在壓縮期間提交一則訊息來重現問題。追蹤佇列訊息堆疊、主要轉錄內容和工作狀態指示器的更新時機。完成的標準是：訊息只保留在佇列中，而轉錄內容和指示器只有在壓縮分派訊息後才會更新。

由索引模型根據 Issue 內容生成。