ci: automate CLI releases

[im10furry]

Summary

• Add a tag-driven GitHub Release workflow for Scion CLI binaries and checksums.
• Update README and docs getting-started pages to be CLI-first and reference the next fixed release version.
• Clean up stale zero-dependency wording so docs match declared external-dependency exceptions.
• Make scion version report unknown instead of dev when no commit metadata is available.

Verification

• go test -count=1 ./...
• go vet ./...
• go run ./cmd/scion doctor --strict
• go run ./internal/cmd/build-bundle; git diff --exit-code -- internal/bundle
• cd docs && npm run build
• Local equivalent cross-build for v0.1.2 assets and SHA256SUMS

Notes

• npm ci reports existing docs dependency audit warnings, but npm run build passes.