fix(config): restore sync.go with warning implementations

Commit 356ab92b deleted internal/config/sync.go while adding
cmd/bd/sync_mode.go, but config.go still references the types and
functions from sync.go, causing build failures.

These serve different purposes:
- internal/config/sync.go: viper-based (config.yaml), has warnings
- cmd/bd/sync_mode.go: storage-based (database), no warnings

Both should coexist. This restores sync.go and sync_test.go from
e82e15a8, and fixes type conversion in sync.go:805.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/sync.go

@@ -802,7 +802,7 @@ func showSyncStateStatus(ctx context.Context, jsonlPath string) error {
 
 	// Sync mode (from config)
 	syncCfg := config.GetSyncConfig()
-	fmt.Printf("Sync mode: %s (%s)\n", syncCfg.Mode, SyncModeDescription(syncCfg.Mode))
+	fmt.Printf("Sync mode: %s (%s)\n", syncCfg.Mode, SyncModeDescription(string(syncCfg.Mode)))
 	fmt.Printf("  Export on: %s, Import on: %s\n", syncCfg.ExportOn, syncCfg.ImportOn)
 
 	// Conflict strategy