feat(tui): add turn summary flash experiment

[kitlangton]

What

Adds an opt-in Turn summary flash TUI experiment. When a live turn completes, the agent, model, and duration summary brightens toward the default text color, then fades over 800ms to each segment's semantic resting color.

The experiment is disabled by default and can be toggled from DevTools → Experiments.

How

• packages/tui/src/component/assistant-summary.tsx owns the production summary rendering and bright-to-rest animation.
• packages/tui/src/routes/session/rows.ts distinguishes live terminal completions from hydrated history, so old turns do not replay the flash when a session opens.
• packages/tui/src/routes/session/index.tsx enables the selected Lingering treatment (800ms at 70% brightness) only when experimental.turn_summary_flash and global animations are enabled.
• packages/tui/src/component/dialog-experiments.tsx registers the experiment in the existing picker.
• packages/tui/AGENTS.md records the local story-first and simulated verification workflow for future TUI experiments, including removing tuning-only scaffolding before commit.

flowchart LR
  A[Live terminal step ends] --> B[Append footer with flash marker]
  B --> C{Experiment enabled?}
  C -- No --> D[Render resting semantic colors]
  C -- Yes --> E[Brighten summary]
  E --> F[Fade for 800ms]
  F --> D
  G[Hydrated session history] --> D

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Scope

• Does not change default TUI behavior.
• Does not animate retry notices or replay animations for historical summaries.
• Does not change mini-mode turn summaries.
• Does not commit the temporary tuning story or dedicated animation tests.

Testing

• bun typecheck in packages/tui
• Push hook: workspace bun turbo typecheck --concurrency=3 (33 packages passed)
• OpenCode Drive isolated simulated turn with experimental.turn_summary_flash: true
• Temporary local tuning story exercised at 100×30 and 60×24 before removal

Demo

Prototype-only tuning story using the production component. The story is not included in this PR, and no real model or session data is involved.

turn-summary-pr.mp4