Expand/collapse earlier messages behind context trim divider #84

Parent

Parent PRD: #82

What to build

Add an expand/collapse toggle to the ContextTrimPointDivider component so users can reveal the full history of messages that were pruned by a /clear or /compact operation. Messages before the trim point are hidden by default but can be shown in a visually dimmed, expandable section.

This slice is UI-only — all data is already in the read model (messages are never deleted, only excluded from the active context window). The toggle just controls whether the pre-trim messages are rendered in the timeline.

Acceptance criteria

• ContextTrimPointDivider gains an expand/collapse chevron button
• Clicking "Show earlier messages" or the expand chevron reveals messages and activities before the trim point
• Revealed messages are visually distinct from active messages (dimmed, or with a distinct background/opacity)
• Clicking "Hide earlier messages" or the collapse chevron hides them again
• Multiple trim points are supported — each divider independently controls its own earlier message section
• Smooth expand/collapse animation (height transition)
• Timeline scroll position is maintained when expanding/collapsing (no jarring jumps)
• Works correctly with streaming messages — if a new message arrives after a trim, the divider stays in the correct position
• Tests cover: expand/collapse render states, multiple dividers, timeline entry derivation with visibility flags

Blocked by

• /clear + /clear N — core context trim infrastructure #83 (core /clear infrastructure)