fix: harden GitHub Actions against supply chain attacks

[CybotTM]

Summary

• Pin all GitHub Actions references to immutable commit SHAs (prevents tag/branch force-push attacks)
• Add Dependabot configuration for automatic GitHub Actions version updates

Context

On 2026-03-19, aquasecurity/trivy-action was compromised via a tag force-push attack that exfiltrated secrets from CI runners. SHA-pinning prevents this class of attack entirely.

The netresearch org now enforces sha_pinning_required=true — workflows using tag/branch references will fail.

Ref: netresearch/ofelia#535

Test plan

• Verify CI passes with SHA-pinned actions
• Verify Dependabot creates PRs for action updates

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the security posture of the project's CI/CD workflows by implementing measures to mitigate supply chain attacks on GitHub Actions. It ensures that all GitHub Actions are referenced by immutable commit SHAs, preventing malicious alterations through tag or branch force-pushes, and introduces automated dependency management for these actions.

Highlights

• GitHub Actions Security Hardening: Pinned all GitHub Actions references to immutable commit SHAs, preventing potential supply chain attacks via tag or branch force-pushes.
• Automated Dependency Updates: Introduced Dependabot configuration to automatically manage and update GitHub Actions versions on a weekly schedule.

Ignored Files

• Ignored by pattern: .github/workflows/** (3)
  • .github/workflows/auto-merge-deps.yml
  • .github/workflows/lint.yml
  • .github/workflows/release.yml

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[gemini-code-assist]

Code Review

This pull request introduces Dependabot configuration to automatically update GitHub Actions, which is a great step towards hardening the CI/CD pipeline against supply chain attacks. My review focuses on refining this configuration. I've suggested splitting the Dependabot groups for GitHub Actions into major and non-major updates. This will help separate potentially breaking changes from routine updates, making the update process smoother and more manageable.