Add shared resolved credential forwarding

internal/adapters/subprocess/credentials.go

@@ -0,0 +1,29 @@
+package subprocess
+
+import (
+	"fmt"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+// CredentialEnvironment returns captured secret values and names for a child
+// invocation. References must already have been resolved by ConfigKit.
+func CredentialEnvironment(credentials []configkit.Secret) (map[string]string, []string, error) {
+	if len(credentials) == 0 {
+		return nil, nil, nil
+	}
+	overrides := make(map[string]string, len(credentials))
+	names := make([]string, 0, len(credentials))
+	for _, credential := range credentials {
+		name := credential.EnvName()
+		value := credential.Value()
+		if !credential.IsResolved() || value == "" {
+			return nil, nil, fmt.Errorf("credential %q must be resolved and nonempty", name)
+		}
+		if _, exists := overrides[name]; !exists {
+			names = append(names, name)
+			overrides[name] = value
+		}
+	}
+	return overrides, names, nil
+}