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 of the repository's CI/CD pipelines by implementing robust measures against supply chain attacks on GitHub Actions and automating the update process for these actions. It ensures that workflows are resilient to external tampering and that dependencies are kept up-to-date.

Highlights

• GitHub Actions Security Hardening: All GitHub Actions references have been pinned to immutable commit SHAs to prevent supply chain attacks, specifically mitigating risks from tag/branch force-push vulnerabilities.
• Automated GitHub Actions Updates: Dependabot configuration has been introduced to automatically manage and update GitHub Actions versions on a weekly schedule.

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[gemini-code-assist]

Code Review

This pull request introduces a dependabot.yml configuration to enable automatic updates for GitHub Actions, which is a valuable security enhancement. My review focuses on making this configuration more robust and maintainable by aligning it with the more detailed examples available within this repository. It's worth noting that while the pull request description mentions pinning actions to commit SHAs, those changes are not present in the current diff. This review is therefore scoped to the dependabot.yml file.