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 project's security posture by implementing critical best practices for GitHub Actions. It addresses potential supply chain vulnerabilities by ensuring that all actions are referenced by their immutable commit SHAs, thereby preventing malicious alterations through tag or branch force-pushes. Additionally, it introduces automation for maintaining these actions by configuring Dependabot to provide weekly updates, ensuring that the CI/CD pipeline remains secure and up-to-date with minimal manual intervention.

Highlights

• GitHub Actions Security Hardening: All GitHub Actions references are now pinned to immutable commit SHAs to prevent supply chain attacks, specifically tag/branch force-push vulnerabilities, aligning with organizational enforcement.
• Automated Dependency Updates: Dependabot configuration has been added to automatically manage and update GitHub Actions versions on a weekly schedule, grouping all action updates.

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[gemini-code-assist]

Code Review

This pull request introduces a Dependabot configuration to keep GitHub Actions updated, which is a key part of hardening against supply chain attacks. The new .github/dependabot.yml file is correctly configured to check for updates weekly and group them into a single pull request. The implementation follows best practices, and I found no issues in the provided changes.