Add GitHub Actions CI workflow

[winrid]

Adds a CI workflow (.github/workflows/ci.yml) that runs on pull requests and pushes to the default branch.

• Installs deps, compiles the committed generated sources (no spec regen in CI), and runs unit + integration + browser tests.
• Integration tests use the FASTCOMMENTS_API_KEY / FASTCOMMENTS_TENANT_ID org secrets.
• Uses pull_request (not pull_request_target), so secrets are not exposed to fork PRs.