ci: update GitHub Actions to Node 24

[gokul-debugger]

Summary

• upgrade GitHub Actions used by CI and publishing to Node 24-native releases
• keep every action pinned to an immutable commit SHA
• preserve existing workflow inputs, permissions, caching, artifact retention, and publishing behavior

Why

The successful v0.1.0 publishing run warned that several Node 20 actions were being forced to run on Node 24. Updating the actions removes that compatibility warning before the next release.

A separate issue is not necessary for this focused maintenance change because the warning identifies both the affected actions and the required migration.

Validation

• verified each tag and immutable SHA against the action's official repository
• confirmed all retained inputs are supported by the selected versions
• parsed both workflow files as YAML
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy src tests
• uv run pytest --cov=contribcheck --cov-report=term-missing (52 passed)