feat(app): show writing command state

[Hona]

Summary

• show Writing command... while shell input is still streaming and has not parsed into a command
• keep Shell shimmering through streamed-input parsing, then make it static when execution starts
• align shell rows to Figma's 28 px compact layout, Inter typography, 6 px spacing, and semantic base/muted colors
• preserve existing command truncation, hover disclosure, expansion, output, and error behavior

Figma: section 1769:236652; writing 1842:10023; command ready 1842:9937; running 1842:10255; hover 1842:10420; expanded 1842:11309.

Testing

• bun typecheck in packages/app, packages/session-ui, and packages/ui
• bun test in packages/session-ui (91 passed)
• bun test in packages/ui (27 passed)
• lifecycle regression suite in Chromium (6 passed)
• tool-projection regression suite in Chromium (6 passed)
• canonical browser transition coverage: tool.input.started -> tool.input.ended -> tool.called
• verified placeholder copy, shimmer state, parsed command replacement, running state, 28 px height, 6 px spacing, Inter 13 px typography, weights, line height, and colors

Performance

The production benchmark was run before and after. Both runs built successfully, then hit the existing blank-app readiness failure before fixture setup; neither emitted metrics.

Known base issue

bun run typecheck:e2e retains the existing error in session-timeline-notices.spec.ts:82: toolPart(...) returns a ToolSeed without the protocol tool part's required time. This PR adds no E2E type errors.

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