test: establish vitest suite, CI, and coverage reporting (44%→83%)

.github/workflows/ci.yml

@@ -0,0 +1,29 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: pnpm/action-setup@v6
+        with:
+          version: 10
+
+      - uses: actions/setup-node@v6
+        with:
+          node-version: 24
+          cache: pnpm
+
+      - run: pnpm install --frozen-lockfile
+
+      - run: pnpm typecheck
+
+      - run: pnpm test
+
+      - run: pnpm build # fail if the bundle no longer builds

.gitignore

@@ -1,5 +1,7 @@
 node_modules/
 dist/
+coverage/
 *.DS_Store
 .claude/
+.vscode/
 better-github.zip

CLAUDE.md

@@ -10,6 +10,22 @@ Use `pnpm`, not npm or yarn.
 pnpm build
 ```
 
+## Test
+
+Vitest, with `happy-dom`. Tests live next to the code as `*.test.ts`.
+
+```sh
+pnpm test            # run once
+pnpm test:watch      # watch mode
+pnpm test:coverage   # run with v8 coverage report (coverage/index.html)
+```
+
+- Coverage thresholds (`vitest.config.ts`) are a **floor to guard against
+  regressions**, meant to be ratcheted up — not a target to pad. They only
+  apply under `--coverage`, so plain `pnpm test` (CI) is unaffected.
+- Prefer tests that assert observable behavior over ones that merely execute
+  code for coverage. Coverage proves code *ran*, not that it was *verified*.
+
 ## Guidelines
 
 - Avoid modifying GitHub's original DOM structure. Prefer appending new elements or using CSS overrides.