feat: unify generated configuration loading and extensible decoders

cmd/go-omni/apigen/upgrade.go

@@ -30,6 +30,39 @@ type UpgradePlan struct {
 	Module, Entry string
 }
 
+// Handwritten old loaders must be migrated explicitly, never silently replaced
+// or left competing with the generated entry point.
+func checkConfigLoadExtension(root string) error {
+	dir := filepath.Join(root, "internal/config")
+	entries, err := os.ReadDir(dir)
+	if errors.Is(err, os.ErrNotExist) {
+		return nil
+	}
+	if err != nil {
+		return err
+	}
+	for _, entry := range entries {
+		name := entry.Name()
+		if entry.IsDir() || !strings.HasSuffix(name, ".go") || strings.HasSuffix(name, "_test.go") || name == "loadGen.go" {
+			continue
+		}
+		rel := "internal/config/" + name
+		if err := safePath(root, rel); err != nil {
+			return err
+		}
+		f, err := goparser.ParseFile(gotoken.NewFileSet(), filepath.Join(root, rel), nil, 0)
+		if err != nil {
+			return fmt.Errorf("parse %s: %w", rel, err)
+		}
+		for _, decl := range f.Decls {
+			if fn, ok := decl.(*ast.FuncDecl); ok && fn.Recv == nil && fn.Name.Name == "Load" {
+				return fmt.Errorf("%s: migrate the project-owned Load function before generating internal/config/loadGen.go; retain Config/Default/Validate and move decoder options to NewLoader; see CONFIG.md", rel)
+			}
+		}
+	}
+	return nil
+}
+
 // PlanUpgrade compares infrastructure against the exact previous release seeds.
 // A custom file is never classified as safe to replace. Logic, middleware,
 // options and repositories outside the changed templates are left untouched.