fix(starter): prefer Node.js 24 with runtime fallbacks

[whywaita]

Summary

The generated runner service script now prefers Node.js 24, allowing it to start with runner distributions that no longer bundle Node.js 20. It falls back to Node.js 20 and then Node.js 16 for older distributions, preserving the compatibility approach from #220.

Validation

• Added five tests that generate the service script through its actual heredoc and execute it with stub runtimes, covering runtime preference, fallback, and service arguments.
• Confirmed the Node.js 24 cases fail before the fix and all cases pass afterward.
• go fmt ./pkg/starter
• go test ./...
• go build ./...
• go vet ./...

CI compatibility

CI failed before tests because the latest Staticcheck requires Go 1.26 while this project uses Go 1.25. Pin Staticcheck to v0.7.0, which supports Go 1.25. Also remove the build-only job's unused reference to a nonexistent metadata step, detected by actionlint.

• actionlint passes.
• Ran pinact run -u; retained existing SHA-pinned action versions to keep unrelated action upgrades out of this change.