Why: A background child should not look like the conversation is busy, and its completion note needs a compact row instead of a full assistant bubble.
Evidence: vitest run passed, including design-guard. tsc -b passed.
Impact: The stop button cancels the conversation run. System notices fold with the shared Collapsible.
Refs: feature/agent-harness-p3b
Co-authored-by: Cursor <cursoragent@cursor.com>
weichenandCursor