feat(dashboard): polish transcript timeline

[christso]

Summary

The transcript tab now behaves like a dense review surface instead of a static message dump. Reviewers can scan the normalized JSONL transcript with first and final messages open, middle messages collapsed, tool calls hidden by default, and joined tool results available on demand.

The UI keeps the transcript artifact contract unchanged: it still renders transcript.jsonl, labels it as normalized JSONL, and leaves raw provider evidence to transcript-raw.jsonl/Files rather than overloading the transcript view.

What changed

• First and final chronological messages expand by default; middle messages can be opened individually.
• Tool calls are collapsed by default, with global expand/collapse controls.
• Joined tool_use.result details remain visible when expanded: arguments, result, error, metadata, status, and duration.
• Transcript actions now distinguish normalized JSONL from raw provider logs.
• The missing-answer warning no longer occupies the top of the transcript when no answer.md artifact exists.

Validation

• bun test ./apps/dashboard/src/components/transcript-timeline.test.tsx
• bunx biome check apps/dashboard/src/components/TranscriptTimeline.tsx apps/dashboard/src/components/transcript-timeline.test.tsx
• git diff --check
• bun --filter @agentv/core build
• bun --filter @agentv/dashboard build
• Browser UAT with agent-browser against a temporary local .agentv/results fixture rendered through the real Dashboard at http://localhost:3127.

Evidence

Private visual evidence is saved in agentv-private:

https://github.com/EntityProcess/agentv-private/tree/evidence/transcript-ui-parity-2026-06-26/transcript-ui-parity-2026-06-26

Relevant AgentV dashboard screenshots are under agentv-dashboard/, alongside the Entire and Vercel reference captures and audit reports.

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