Add Theme Preview plugin

[brsbl]

What changed

This adds Theme Preview, a nav-panel plugin for iterating on a live theme in a split beside an agent. The additive finishing commits complete and harden that workflow:

• a background fs.watch service publishes theme-preview:changed; the app refreshes with useRealtime, while an active-file mtime check re-applies changed custom and plugin themes
• stale catalog work cannot revert a newer picker selection, and out-of-order frontend responses cannot overwrite its state
• plugin-contributed theme CSS resolves through sdk.plugins.list() plus each manifest's bb.themes[].css
• partial themes inherit the base palette in both modes, shared :root declarations apply to both, and local var() references and fallbacks resolve before rows render
• bb's six bundled palettes show real chips extracted from bb source at c942421a4
• the mock is the main stage, with a compact view toggle and a right-hand style-guide rail topped by one combined theme/mode picker
• Ink and Status rows show alpha-composited WCAG contrast ratios against the rendered surface
• the bundled bb-theme-authoring skill documents the split workflow, theme file shape, tokens, application, and verification

The plugin stays on the released @get-bb/plugin-sdk 0.4.8 toolchain and retains engines.bbPluginSdk: ^0.4.1 and engines.bb: >=0.0.34.

Verification

• npm run check --workspace=plugins/theme-preview — typecheck, build, 2 files / 10 tests
• node tooling/check-repository.mjs — repository hygiene passed
• Exact-head CI run 32528269658 fully green
• Exact-head install in the desktop dev fixture
• Mobile, narrow, normal desktop, and ultrawide captures inspected, plus dark mode and transition widths
• Concurrency, translucent contrast, partial theme, shared root, and derived-variable regression tests pass

Evidence

These are renderer captures from the same desktop dev fixture, route, 1440×900 viewport, and device scale factor. Native window chrome is absent because the bb host did not have Screen Recording permission for this run.

Before — parent behavior on origin/main, plugin unavailable

After — exact PR head e07166b9f4f31de0883fb0a7be1648e96e938b24, live mock and inspector rail

Responsive, dark-mode, transition, and functional evidence

BB-Thread-ID: thr_yqw7fnuxf6