chore(v0): drop html coverage reporter in CI

The coverage.reporter list was ['text', 'json', 'html']. CI never
downloads the generated HTML coverage site - it's written to the
runner and thrown away on job completion. Dropping it saves ~0.5-2s
per CI test job and simplifies the coverage output directory.

Kept:
- text: printed to the CI log for quick pass-rate glances
- json: consumed by codecov/codecov-action@v5, required for upload

Local developers who want an HTML report can run:
  pnpm test:run --coverage --coverage.reporter=html
ad-hoc - this config affects CI only.

Trivial cleanup. Independent of the pool: vmThreads change
(7943d5d) - each can be reverted separately.

Stage 2 of v0 test pool spec.

Author: johnleider

vitest.config.ts

@@ -8,7 +8,7 @@ export default defineConfig({
     testTimeout: 20_000,
     coverage: {
       provider: 'v8',
-      reporter: ['text', 'json', 'html'],
+      reporter: ['text', 'json'],
       include: ['packages/0/src/**/*.ts', 'packages/0/src/**/*.vue'],
       exclude: [
         '**/*.test.ts',