fix(loader): disable config file rewrite during startup

* migrate legacy schemas in memory only
* log hint to use amcs-migrate-config for persistence

internal/config/loader.go

@@ -40,16 +40,14 @@ func LoadWithLogger(explicitPath string, log *slog.Logger) (*Config, string, err
 	}
 
 	if len(applied) > 0 {
-		if err := rewriteConfigFile(path, data, raw); err != nil {
-			return nil, path, err
-		}
 		if log != nil {
 			for _, step := range applied {
-				log.Warn("config migrated",
+				log.Warn("config migrated in memory",
 					slog.String("path", path),
 					slog.Int("from_version", step.From),
 					slog.Int("to_version", step.To),
 					slog.String("describe", step.Describe),
+					slog.String("hint", "persist with amcs-migrate-config"),
 				)
 			}
 		}
@@ -81,22 +79,6 @@ func decodeTyped(raw map[string]any) (Config, error) {
 	return cfg, nil
 }
 
-func rewriteConfigFile(path string, original []byte, migrated map[string]any) error {
-	backupPath := fmt.Sprintf("%s.bak.%d", path, time.Now().Unix())
-	if err := os.WriteFile(backupPath, original, 0o600); err != nil {
-		return fmt.Errorf("write backup %q: %w", backupPath, err)
-	}
-
-	out, err := yaml.Marshal(migrated)
-	if err != nil {
-		return fmt.Errorf("marshal migrated config: %w", err)
-	}
-	if err := os.WriteFile(path, out, 0o600); err != nil {
-		return fmt.Errorf("write migrated config %q: %w", path, err)
-	}
-	return nil
-}
-
 func ResolvePath(explicitPath string) string {
 	if path := strings.TrimSpace(explicitPath); path != "" {
 		if path != ".yaml" && path != ".yml" {

internal/config/loader_test.go

@@ -3,6 +3,7 @@ package config
 import (
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 )
@@ -215,7 +216,15 @@ logging:
 	if err != nil {
 		t.Fatalf("glob backups: %v", err)
 	}
-	if len(entries) != 1 {
-		t.Fatalf("backup files = %d, want 1", len(entries))
+	if len(entries) != 0 {
+		t.Fatalf("backup files = %d, want 0 (load should not rewrite config)", len(entries))
+	}
+
+	originalOnDisk, err := os.ReadFile(configPath)
+	if err != nil {
+		t.Fatalf("read original config: %v", err)
+	}
+	if !strings.Contains(string(originalOnDisk), "provider: \"litellm\"") {
+		t.Fatalf("expected source config to remain unchanged on disk")
 	}
 }