Writable GitHub Actions CI that runs typecheck + Biome + tests on PRs

[jstuart0]

Goal

Right now CI is manual ("I ran `bun test` locally"). Let's have GitHub Actions do it for us on every PR.

Scope

• Create `.github/workflows/ci.yml`.
• Steps:
  1. Checkout.
  2. Install Bun via `oven-sh/setup-bun@v2`.
  3. `bun install --frozen-lockfile`.
  4. `bunx biome check src/`.
  5. `bun run typecheck`.
  6. `bun test`.
  7. `bun run build` (catches Vite errors).
• Cache `~/.bun/install/cache` between runs (bun's global cache).
• Fail the workflow if any step fails.

Optional follow-ups

• Add a status badge to README.
• Require the check to pass before merging to `main` (GitHub branch protection rule — needs repo admin, can be done after the workflow is in).

Acceptance criteria

• `.github/workflows/ci.yml` runs green on a fresh PR touching `src/`.
• Fails loudly when you intentionally introduce a typecheck error in a test PR.