APPSEC-553: Add README, Semgrep OSS, and scan for secrets workflows

[mantej]

Summary

• Adds a README describing the repository's purpose
• Adds a Semgrep OSS GitHub Actions workflow that runs on PRs, push to main, and a weekly schedule — no token required, findings upload to GitHub Code Scanning

Test plan

• Verify Semgrep workflow triggers on PR and findings appear in the Security tab

[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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