feat: add CodeQL SAST analysis workflow for C#

[willvelida]

Summary

Adds GitHub CodeQL static analysis (SAST) for C# to the CI/CD pipeline, addressing ASI04 §4.5 (Supply Chain Vulnerabilities — Static Analysis) from the OWASP Agentic Top 10 implementation plan.

Changes

• New: .github/workflows/codeql.yml — CodeQL analysis workflow

Details

The workflow:

• Triggers: push to main, PRs to main, and weekly schedule (Monday 06:00 UTC)
• Permissions: minimal — actions: read, contents: read, security-events: write
• Steps: checkout → setup .NET 10 SDK → CodeQL init (C#) → autobuild → analyze
• Results appear in the GitHub Security tab

References

• Implementation plan: docs/plans/2026-03-13-asi04-codeql-sast.md
• Parent plan: docs/plans/2026-03-13-owasp-agentic-top10-implementation-plan.md §4.5
• Blog post: docs/blog-post-ideas/06-owasp-asi04-supply-chain-vulnerabilities.md

[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.