feat(M38): SSH target connector for agentless deployment via SSH/SFTP

Adds a new target connector enabling certificate deployment to any
Linux/Unix server without installing the certctl agent binary. Uses the
proxy agent pattern — a single agent in the same network zone deploys
certs to remote servers over SSH/SFTP.

Key additions:
- SSH/SFTP connector with key auth (file/inline) + password auth
- Injectable SSHClient interface for cross-platform testing (25 tests)
- Shell injection prevention via validation.ValidateShellCommand()
- Configurable cert/key/chain paths with octal permissions
- GUI: 11 SSH config fields in target create wizard

Also fixes pre-existing frontend bug where all target type strings
(nginx, apache, etc.) were sent as lowercase but the backend expects
proper-case (NGINX, Apache, etc.), breaking GUI-created targets.
Adds missing TargetTypeSSH to validTargetTypes service map.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/agent/main.go

@@ -31,6 +31,7 @@ import (
 	"github.com/shankar0123/certctl/internal/connector/target/caddy"
 	"github.com/shankar0123/certctl/internal/connector/target/envoy"
 	pf "github.com/shankar0123/certctl/internal/connector/target/postfix"
+	sshconn "github.com/shankar0123/certctl/internal/connector/target/ssh"
 	"github.com/shankar0123/certctl/internal/connector/target/f5"
 	"github.com/shankar0123/certctl/internal/connector/target/haproxy"
 	"github.com/shankar0123/certctl/internal/connector/target/iis"
@@ -647,6 +648,15 @@ func (a *Agent) createTargetConnector(targetType string, configJSON json.RawMess
 		}
 		return pf.New(&cfg, a.logger), nil
 
+	case "SSH":
+		var cfg sshconn.Config
+		if len(configJSON) > 0 {
+			if err := json.Unmarshal(configJSON, &cfg); err != nil {
+				return nil, fmt.Errorf("invalid SSH config: %w", err)
+			}
+		}
+		return sshconn.New(&cfg, a.logger)
+
 	default:
 		return nil, fmt.Errorf("unsupported target type: %s", targetType)
 	}