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 repository's CI/CD workflows by implementing best practices against supply chain attacks. By enforcing SHA-pinning for all GitHub Actions, it mitigates risks associated with malicious tag or branch force-pushes. Additionally, the introduction of Dependabot for GitHub Actions ensures that all workflow dependencies are automatically kept up-to-date, further reducing potential security vulnerabilities and maintenance overhead.

Highlights

• Security Hardening: Pinned all GitHub Actions references to immutable commit SHAs to prevent tag/branch force-push attacks, addressing a known vulnerability class.
• Automated Updates: Added Dependabot configuration for automatic GitHub Actions version updates, ensuring dependencies remain current and secure.

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[gemini-code-assist]

Code Review

This pull request adds a Dependabot configuration to manage updates for GitHub Actions. While this is a good practice for keeping actions up-to-date, the repository already contains a renovate.json file. The default Renovate configuration also handles GitHub Actions updates, which means you might get duplicate pull requests from both Dependabot and Renovate. It is recommended to consolidate on a single tool for this task to avoid confusion.

Furthermore, the pull request description states that GitHub Actions have been pinned to commit SHAs, but these changes are not reflected in the provided diff. This might be an oversight and could cause the CI to fail if SHA pinning is enforced at the organization level as mentioned.