Add zizmor workflow for GitHub Actions static analysis

[david-wagih]

Summary

Adds a zizmor workflow that performs static analysis of the repository's GitHub Actions workflows to detect security issues (template injection, excessive GITHUB_TOKEN permissions, credential persistence, unpinned actions, etc.).

• Runs on push and pull_request to master.
• Uploads results as SARIF to GitHub Code Scanning so findings surface in the Security tab and on PRs.
• Findings are reported but do not fail the build, so this is non-disruptive to merge.
• Top-level permissions: {} with least-privilege job permissions; the action and actions/checkout are pinned to commit SHAs and check out with persist-credentials: false.

This mirrors the same zizmor scanning already adopted in other G-Research repositories (e.g. consuldotnet, common-actions, NuPerfMonitor).

🤖 Generated with Claude Code

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

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• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.