feat(tui): auto-collapse reasoning blocks only when the reasoning phase ends

[JElfferich]

Summary

• display.sections.thinking: collapsed now behaves as an auto preference: the TUI keeps the live reasoning panel open while reasoning streams, and collapses it the moment the reasoning phase ends (first tool call, final answer, or new turn)
• Sealed reasoning segments from earlier phases in the same turn stay collapsed — only the currently-streaming block expands
• expanded / hidden / MoA-reference panel semantics are unchanged

Motivation

With thinking: collapsed, the TUI collapsed every reasoning block — including the one currently being generated — so users couldn't watch reasoning stream while keeping finished blocks tidy. There was no mode for "expanded while live, collapsed when done". This PR makes collapsed that mode.

Changes

• ui-tui/src/app/turnController.ts — syncReasoningSegment() tags the open segment isLiveReasoning; endReasoningPhase() / closeReasoningSegment() seal the tag when reasoning ends
• ui-tui/src/components/streamingAssistant.tsx — passes reasoningActive only to the live segment (block.msg.isLiveReasoning), not to every block
• ui-tui/src/components/thinking.tsx — ToolTrail auto-opens while reasoningActive under collapsed mode; expanded/hidden/MoA references keep manual/current semantics
• ui-tui/src/components/messageLine.tsx — threads reasoningActive through to ToolTrail
• ui-tui/src/types.ts — new Msg.isLiveReasoning field
• ui-tui/src/__tests__/thinkingLiveCollapse.test.tsx — new tests: open-on-stream, collapse-on-finish (incl. mid-turn rerender), expanded-mode no-op

Test Plan

• npm run typecheck passes
• npm run build passes
• New thinkingLiveCollapse.test.tsx (4 tests) + existing details, messageLine, thinkingMoaReferenceVisibility, blockLayout, queueSubmission suites all pass
• Full ui-tui suite: 0 new failures vs baseline (21 pre-existing failures in subscriptionOverlay/ink-backpressure/appChromeBlockedTimers — unrelated, A/B-verified on unpatched HEAD)
• Manual: reasoning-heavy prompt in TUI — panel opens while streaming, collapses when reasoning ends, only current block expands

Notes for Reviewers

• The isLiveReasoning flag approach was chosen over "last thinking segment" heuristics because the turn controller already tracks the open segment index precisely, and the flag survives segment merging (mergeToolShelfInto spreads the target msg).
• Behavior is gated on the existing display.sections.thinking: collapsed config value — no new config keys.

Semantics change (reviewer note)

display.sections.thinking: collapsed previously meant "always collapsed". This PR redefines it as an auto preference: expanded while reasoning is live, collapsed when the reasoning phase ends. The strict "never expanded" behavior is still reachable via display.sections.thinking: hidden, and manual chevron clicks mid-stream still win (the auto-open only re-applies when the reasoning phase changes). Alternative considered: a new auto mode value leaving collapsed strict — rejected to avoid a second knob for the same concept; happy to split if maintainers prefer.

[teknium1]

Thanks for this fix! It was salvaged into #86771 (cherry-picked onto current main with your authorship preserved in the commit history) and is now merged. Closing since the work has landed.