P1: Thinking stream reshuffles feed content and fights scroll position

[bleuropa]

Summary

Thinking stream updates reshuffle feed content, making it hard to read other cards while an agent is thinking.

Root Cause

In workspace_live.ex:886-931, every agent_stream_delta message finds the existing thinking event by ID and updates it in-place via List.update_at/3. The content keeps growing while new events from other agents append below — forcing the user to scroll back up constantly. The ScrollToBottom hook makes it worse by fighting against the user's scroll position.

Scope

• Decouple thinking stream updates from feed reflow
• Fix or conditionally disable ScrollToBottom when user has scrolled up manually
• Consider collapsing/truncating thinking content in the feed with "see full" expansion

Acceptance Criteria

• Thinking stream updates don't cause visible content jumps for other cards
• User scroll position is preserved when manually scrolled up
• Feed remains auto-scrollable when user is at the bottom