Add GitHub Actions CI pipeline

[c99e]

Summary

The repo has no automated CI. Contributors can break tests or types without knowing until someone runs checks locally. A GitHub Actions workflow should gate PRs on the project's existing quality checks (bun test, tsc --noEmit) and be structured to accommodate future checks like linting and Node version compatibility.

Acceptance criteria

• PRs and pushes to main trigger a CI workflow
• CI fails if any unit test fails
• CI fails if type checking reports errors
• The workflow is extensible — adding a new check (e.g., lint) should mean adding one step, not restructuring the pipeline
• CI runs against multiple Node.js versions, since the extension runs inside pi's Node runtime (not Bun)
• A CI status badge is added to the README

Testing guidance

Open a PR with a deliberate type error or failing test and confirm CI catches it.

Out of scope

• Adding a linter — that's a separate decision. CI should be structured so a lint step slots in naturally later.
• Release automation / publishing
• Integration tests against live GraphQL endpoints