Add security cleanup documentation

SECURITY_CLEANUP.md

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+# Security Cleanup - April 9, 2026
+
+## What happened
+Sensitive data (code exports containing potential tokens) was accidentally committed to git history in commit 2807a55.
+
+## Actions taken
+1. ✅ Removed sensitive files from tracking (code_export.txt, code_export.ps1.test.txt)
+2. ✅ Added proper .gitignore to prevent future leaks
+3. ✅ Used git-filter-repo to completely remove files from entire git history
+4. ✅ Prepared for force-push to rewrite remote history
+
+## Next steps (MANUAL)
+1. **CRITICAL**: Revoke the exposed Gitea token immediately in Gitea settings
+2. Generate a new Gitea token
+3. Update any services using the old token
+4. Execute: `git push --force origin master` to rewrite remote history
+5. Notify any collaborators to re-clone the repository
+
+## Prevention
+- .gitignore now blocks code_export.txt and similar files
+- export_code.ps1 already has exclusions for sensitive patterns
+- Always review git status before committing
+
+## Verification
+After force-push, verify that sensitive files are not in history:
+```bash
+git log --all --full-history -- code_export.txt
+# Should return nothing
+```