Include agent_system_identity in storage-migrate copyOrder

Why: TestCopyOrderCoversAllFixtureTables fails when new schema tables
are omitted from the migrate tool inventory.
Evidence: go test ./cmd/storage-migrate/ -run CopyOrder
Impact: copyOrder gains agent_system_identity (no FK deps).
Refs: U1 I3; Backend PR #170

Co-authored-by: Cursor <cursoragent@cursor.com>

cmd/storage-migrate/main.go

@@ -291,6 +291,8 @@ var copyOrder = []string{
 	"secret_keys", "settings_snapshots", "regen_pending", "context_entries",
 	"user_commands", "pending_notices", "recovery_holds",
 	"surfaces", "surface_revisions", "surface_actions", "plugin_activation",
+	// U1 I3：服务实例 DB 身份（无外键；迁移须保留以免目标库丢锚）
+	"agent_system_identity",
 }
 
 const controlDDL = `