feat: add SARIF output and GitHub Code Scanning upload to self-scan workflow

[sonukapoor]

Adds sarif: "true" to the self-scan action job and uploads the result to GitHub Code Scanning via github/codeql-action/upload-sarif@v3.

if: always() on the upload step ensures findings are surfaced even when the scan exits non-zero due to --fail-on high.

Closes #346

[github-advanced-security]

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