- go.mod module path: usher → github.com/nexustar/usher (enables
  go install github.com/nexustar/usher/cmd/usher@latest)
- all 44 internal import paths updated to match
- Version: const → var, injected via -ldflags -X at build time
  (Makefile uses git describe, release workflow uses the tag)
- .github/workflows/ci.yml: vet + test on push/PR
- .github/workflows/release.yml: cross-compile 4 targets on v* tag,
  upload binaries + checksums.txt to GitHub Release
- README: add Install section with release link and go install

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>