ci: adopt shared CI tooling

[bytaesu]

Merge this PR after merging #36

[greptile-apps]

Greptile Summary

This PR adopts centrally maintained CI tooling for package setup and GitHub Actions validation.

• Replaces manual Node, pnpm, and dependency-install steps with the pinned shared setup-pnpm action.
• Restricts checkout credentials and workflow permissions.
• Adds reusable workflows for GitHub Actions linting, zizmor checks, and code-scanning uploads.

Confidence Score: 5/5

The PR appears safe to merge because no blocking failure remains.

No blocking failure remains.

_{Reviews (2): Last reviewed commit: "Merge branch 'main' into 2026-08-31/ci/a..." | Re-trigger Greptile}