Add GitHub Actions CI for tests, typecheck, and build

[GaspardKirira]

The project currently has useful package scripts for validation:

• bun test
• bun run typecheck
• bun run build

It would be helpful to add a GitHub Actions workflow so pull requests automatically run the test suite, TypeScript type checking, and the build step.

This would make contributions safer and help maintainers review changes with more confidence.

Suggested workflow:

• run on pull requests and pushes to main
• setup Bun
• install dependencies with bun install
• run bun test
• run bun run typecheck
• run bun run build

This would be especially useful for external contributors because every PR would show whether the project still passes the expected checks.