Publish native Draftcat CLI through npm

[renezander030]

Summary

• add the public draftcat npm package with a native CLI launcher
• download version-matched GitHub release binaries for Linux, macOS, and Windows on x64 and arm64
• verify downloaded archives against the release SHA256SUMS file
• build and attach native binaries automatically for version tags
• validate the npm package in CI and document npm install -g draftcat

Validation

• npm test
• npm pack --dry-run
• go test -count=1 -short -timeout 60s ./...
• go vet ./...
• cross-build all six supported platform and architecture targets
• git diff --check