feat(desktop): show context compaction in the task transcript

[Astro-Han]

Problem

Desktop currently presents context compaction only through a running toast followed by a terminal toast. Once it disappears, the task transcript has no visible record that compaction ran or how it ended.

Desired outcome

Show a lightweight inline system row in the chat transcript that moves from compacting to compacted, unchanged, or failed. It should project the existing Runtime compaction Turn and typed outcome rather than introduce a second renderer state machine.

Alternatives or workarounds

#3545 adds immediate running and terminal Toast feedback. This issue tracks the durable in-context presentation only.

[liuxiaocs7]

Hi, @Astro-Han, I want to have a try, could assign this issue to me, thanks!

[Astro-Han]

Assigned!