Renumber agent_system_identity to v27 after login_keys took v26

Why: main #171 claimed migration v26; rebase conflict required
bumping the U1 I3 table and completing copyOrder for both new tables.
Evidence: go test ./internal/store/ ./cmd/storage-migrate/ -run
'SystemIdentity|CopyOrder'
Impact: migrate v27; copyOrder += login_keys + agent_system_identity.
Refs: Backend PR #170; conflict with main 1d9e8f4

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",
+	// v26 login_keys（#171 合入 main 时漏登记；本 PR 一并补齐）
+	"login_keys",
 	// 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",
 }

internal/store/system_identity_test.go

@@ -55,13 +55,13 @@ func TestSystemIdentityMigrationCreatesTable(t *testing.T) {
 	}
 	found := false
 	for _, v := range versions {
-		if v == 26 {
+		if v == 27 {
 			found = true
 			break
 		}
 	}
 	if !found {
-		t.Fatalf("migration v26 not applied: %v", versions)
+		t.Fatalf("migration v27 not applied: %v", versions)
 	}
 	var name string
 	if err := st.DB().QueryRow(`SELECT name FROM sqlite_master WHERE type='table' AND name='agent_system_identity'`).Scan(&name); err != nil || name != "agent_system_identity" {