Establish system identity before the multi-instance account bootstrap

Why: bootstrap-admin now refuses a database that has no agent_system_identity row, and serve does not insert one.
Evidence: Dev CI run 3266 job 5008 failed with system identity missing on 7eef723.
Impact: test setup only.
Refs: Agent-System-Dev pulls/558
Co-authored-by: Cursor <cursoragent@cursor.com>

tests/integration/multi-instance/multi_instance_test.go

@@ -438,6 +438,8 @@ func (e *env) killNine(t *testing.T, inst *instance) {
 // alice 发放激活凭据。凭据兑换后设置 alicePass，然后才能发 API Key。
 func (e *env) loginAlice(t *testing.T) *userClient {
 	t.Helper()
+	// 账户事务会锁库内服务身份；serve 不补写这行，须先显式建立。
+	e.cli(t, "identity", "ensure", "--dsn", e.dbDSN, "--instance-id", "mi-acceptance", "--dbref", "pg")
 	e.cli(t, "user", "bootstrap-admin", "--dsn", e.dbDSN, "--owner", "mi_setup", "--name", "Setup",
 		"--out-file", filepath.Join(t.TempDir(), "setup-grant"))
 	grantFile := filepath.Join(t.TempDir(), "alice-grant")