Add CI optimizations and README badges

Improve CI workflow and add status badges to README:

**CI optimizations:**
- Add Go module caching
- Add concurrency groups to cancel outdated runs
- Pin action versions for reproducibility

**README badges:**
- CI status badge
- Go version badge
- License badge

Inspect the repository's CI workflow and README first. Check how Go dependencies are currently installed and how the README presents project metadata; done means the workflow uses caching, concurrency, and pinned actions, while the README displays working CI, Go version, and license badges.

Written by the indexing model from the issue text.