Why: After a refresh the timeline had no thinking duration and successful tool rows expanded to nothing, because only failures stored a summary.
Evidence: go test ./internal/runtime/ -count=1 -run 'TestLLMRunnerRecordsThinkingAndToolDuration|TestLLMRunnerToolTurnTextIsProcess|TestLLMRunnerDeliverablePinnedAfterText'
Impact: Thinking parts gain durationMs; tool calls gain durationMs; tool results keep a short success summary. Requires the core duration field.
Refs: feature/chat-timeline-preview
Co-authored-by: Cursor <cursoragent@cursor.com>