ci: bump actions/checkout and actions/setup-node majors for Node 24

[Codeturion]

GitHub is forcing all actions still on Node 20 to run under Node 24 starting June 2, 2026. See: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Both pinned actions used node20 at v4 and have been updated to use node24 natively at v6.

Bumps:

• actions/checkout@v4 → actions/checkout@v6 (uses node24)
• actions/setup-node@v4 → actions/setup-node@v6 (uses node24)

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Generated by Claude Code

[gemini-code-assist]

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