ci: migrate GitHub Actions to Node24-compatible runtimes (NEXO-507)

[flitzrrr]

This PR implements the NEXO-507 GitHub Actions Node.js 24 migration scope for .github.

Scope covered

• Performed repository-level workflow inventory.
• Upgraded GitHub Actions references to Node24-compatible variants where needed.
• Added a recurring guard workflow to detect regressions (node16 / node20 runtimes).

Workflow changes in this repository

Removed legacy references

• None

Added/updated references

• actions/checkout@v6

Special migration notes

• No special workflow rewrites were required beyond version upgrades and guard rollout.

Files touched

• .github/workflows/node-runtime-guard.yml

Guard workflow added

• .github/workflows/node-runtime-guard.yml
• Triggered on PR/push workflow changes, weekly schedule, and manual dispatch.
• Resolves action metadata and fails if runs.using is node16 or node20.

Validation

• All modified workflow files parse as valid YAML.
• Post-change scan in this repository reports 0 legacy Node action runtimes.

Refs NEXO-507.

[flitzrrr]

Closing this PR as part of NEXO-507 scope correction. This PR only introduced the guard workflow without a repository-specific Node20->Node24 migration delta.