fix(web): render working state as turn header

[rynfar]

Problem

Pylon adopted upstream's active-turn header ordering but kept its older live-tail Working marker and composer-clearance padding. The animated marker therefore appeared in the middle of streamed assistant output and made the timeline look out of order.

Fix

• render Working with the same divider-header surface as current T3 upstream
• remove the obsolete live-tail DotMatrix and bottom padding
• cover the rendered user → Working → assistant order and header styling

The timeline derivation and intentional subagent launch anchors are unchanged.

Verification

• vp test run --dir apps/web/src/components/chat MessagesTimeline.test.tsx MessagesTimeline.logic.test.ts — 80 passed
• vp run -F @t3tools/web typecheck
• vp fmt --check apps/web/src/components/chat/MessagesTimeline.tsx apps/web/src/components/chat/MessagesTimeline.test.tsx
• targeted vp lint — no errors; one existing unused-import warning
• integrated web pass with isolated state: verified streamed assistant/tool content and the fresh-send Thinking state

Evidence

Before

After

Model: openai-codex/gpt-5.6-sol
Harness: Prime Agent

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.3 KiB	13.5 KiB	+196 B (+1.4%)	15.1 KiB	✅
Codex	Thread snapshot wire	6.9 KiB	6.9 KiB	−10 B (−0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.4 KiB	6.6 KiB	+206 B (+3.2%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	55.5 KiB	57.0 KiB	+1.5 KiB (+2.7%)	66.4 KiB	✅
Codex	Live turn messages	9	11	+2 (+22.2%)	21	✅
Claude	Total thread wire	13.3 KiB	13.3 KiB	+2 B (+0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	6.9 KiB	6.9 KiB	−10 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.4 KiB	6.4 KiB	+12 B (+0.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	56.3 KiB	56.3 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: 1da5a39 · PR result: b57f35a · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 109.4 KiB
• Claude decoded thread snapshot: 110.0 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.