feat: add security scrubbing skill documentation.

.agent/skills/security_scrubbing/SKILL.md

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+# Security Scrubbing Skill
+
+This skill provides a standardized workflow for identifying and removing sensitive information (passwords, tokens, keys) from Git history using `git filter-repo` and `gitleaks`.
+
+## Guidelines
+
+1. **Identification**: Always run `gitleaks detect -v` first to identify the scope of exposed secrets.
+2. **Replacement Plan**: Create a mapping file (e.g., `expressions.txt`) using the format `old_value==>new_value`.
+3. **Execution**: Use `git filter-repo --replace-text expressions.txt --force` to rewrite history.
+4. **Verification**: Re-run `gitleaks` to ensure zero leaks remain.
+5. **Synchronization**: Force-push to all remote remotes (`git push origin <branch> --force`). Update all collaborators.
+6. **Archiving**: Log the operation in the project's Runbook directory with a timestamped record.
+
+## Best Practices
+
+- Always use descriptive placeholders like `YOUR_PASSWORD` or `MFA_SECRET_PLACEHOLDER`.
+- Scan all branches, not just the active one, if secrets were historically committed elsewhere.
+- Notify the team immediately after a force-push as it breaks local clones.