Migrate to a Vite plugin architecture (`vite-plugin-evalite`)

[pattobrien]

With Vitest 3.1+'s configureVitest plugin hook, it's possible to integrate Evalite directly as a Vite plugin rather than requiring a separate evalite.config.ts and custom CLI (evalite watch, evalite run).

A vite-plugin-evalite could register the reporter, configure test patterns for *.eval.ts, and spin up the UI server—all through standard Vite/Vitest primitives. Users would just add the plugin to their existing vite.config.ts and run vitest as normal. This would reduce surface area, eliminate the duplicated CLI, and let Evalite compose naturally with existing Vite/Vitest setups.