feat: add CodeQL security scanning workflow

[PAMulligan]

Summary

• Adds .github/workflows/codeql.yml for automated security vulnerability detection
• Configured for JavaScript/TypeScript analysis using GitHub CodeQL
• Runs on push to main, pull requests to main, and weekly (Monday 6am UTC)

Test plan

• Verify workflow appears in the Actions tab after merge
• Confirm CodeQL analysis completes on a PR targeting main
• Check Security tab for CodeQL alerts after first run

Closes #13

🤖 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.

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.