Add CI (GitHub Actions): run tests + typecheck on every push/PR

[infralayers]

Goal

Right now nothing verifies npm test / npm run typecheck stay green except a human running them locally. A CI badge is also a real, visible OSS-credibility signal for the repo.

Steps

1. Add .github/workflows/ci.yml that, on every push and PR to main:
   • checks out the repo
   • sets up Node (use the same major version as local dev: Node 20)
   • runs npm install
   • runs npm run typecheck
   • runs npm test
2. Push a branch and open a throwaway PR to confirm the workflow actually runs and reports status
3. Add the workflow status badge to the root README.md

Definition of done

A PR shows a green (or correctly red, if you intentionally break something to test) CI check. Badge visible in README.

Suggested owner: good starter task for whoever wants something more mechanical/lower-risk first — no MV3 domain knowledge needed.

[Premnath-06]

I'd like to work on this. I'll add a GitHub Actions workflow to run typecheck and tests on every push and pull request, then update the README with the workflow status badge.