ci: update GitHub Actions to Node 24-compatible versions

[CoderDeltaLAN]

Summary

• Update actions/checkout from v4 to v6.
• Update actions/setup-python from v5 to v6.
• Keep the required check name unchanged: local-checks / Python 3.12.

Scope

• CI workflow only.
• No source code, tests, docs, packaging, release, branch protection, or dependency changes.

Local verification

• ./scripts/check.sh passed.
• 89 tests passed.
• Diff is limited to .github/workflows/ci.yml.
• v0.1.0 tag/release was verified intact before push.