v1.2.6

Add full mTLS support for agent connections

Implement complete mutual TLS authentication for agent-to-server connections:
- Add certificate generation utilities in certgen package
- Generate client certificates when creating agent hosts via API
- Add endpoint to download client certificates with auth token verification
- Update setup script to automatically download and configure client certs
- Configure systemd/launchd services to use client certificates
- Add ClientCAKey configuration option for certificate generation

This ensures secure, authenticated connections between agents and the control
plane when mTLS is enabled in the server configuration.