ci: add xgrep security scan workflow

[tas50]

Adds the standard xgrep security-scan workflow so this repo matches the other
cinc-* projects (reference: cinc-console).

The workflow runs mondoohq/actions/xgrep
on pushes to main and on pull requests, then uploads results to the Security
tab via SARIF. It is report-only (fail-on: off) to start — findings surface
without blocking merges; tighten to error once the baseline is triaged.

All action refs are pinned to commit SHAs, matching the source workflow.

[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.

What Enabling Code Scanning Means:

• 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.