Add CI workflow with cargo test, fmt, and clippy

[sunilp]

Summary

The project has no CI. Add a GitHub Actions workflow that runs on PRs to ensure code quality.

What to do

1. Create .github/workflows/ci.yml
2. Include these checks:
   • cargo fmt --check — formatting
   • cargo clippy -- -D warnings — linting
   • cargo test — test suite
3. Run on: push to main, pull requests to main

Notes

• The project currently has a JamJet path dependency (../jamjet/runtime/). The CI workflow may need to handle this — either by mocking the dependency or skipping until standalone mode is complete. Document any workaround needed.

Acceptance criteria

• CI runs on PRs
• Formatting, linting, and tests are checked
• Badge added to README (optional)