Implement Phase 3 of Custom DNS Provider Plugin Support with comprehensive
security controls for external plugin loading.
Add CHARON_PLUGIN_SIGNATURES env var for SHA-256 signature allowlisting
Support permissive (unset), strict ({}), and allowlist modes
Add directory permission verification (reject world-writable)
Configure container with non-root user and read-only plugin mount option
Add 22+ security tests for permissions, signatures, and allowlist logic
Create plugin-security.md operator documentation
Security controls:
Signature verification with sha256: prefix requirement
World-writable directory rejection
Non-root container execution (charon user UID 1000)
Read-only mount support for production deployments
Documented TOCTOU mitigation with atomic deployment workflow
