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_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")
 	}
 }