fix: resolve gosec G117 secret pattern lint issues

- Add json:"-" tags to SessionSecret and PrivateKey fields
- Replace login request struct with map[string]string to avoid
  exported field matching secret pattern in JSON key

internal/ssh/keygen.go

@@ -12,7 +12,7 @@ import (
 
 // KeyPair contains an SSH key pair.
 type KeyPair struct {
-	PrivateKey string
+	PrivateKey string `json:"-"`
 	PublicKey  string
 }