feat(drainage): execute frontend actions

features/agent/api/client.ts

@@ -54,6 +54,8 @@ export type AgentSessionStreamDataEventType =
   | "question_request"
   | "question_response"
   | "tool_call"
+  | "frontend_action"
+  | "frontend_action_result"
   | "ui_envelope"
   | "session_title"
   | "done"
@@ -93,6 +95,8 @@ export type AgentApiClient = {
   deleteSession: (sessionId: string) => Promise<void>;
   getModels: () => Promise<AgentModelsResponse>;
   getUiRegistry: () => Promise<unknown>;
+  getFrontendActionRegistry: () => Promise<unknown>;
+  submitFrontendActionResult: (sessionId: string, actionId: string, result: unknown) => Promise<void>;
   resolveRenderRef: (renderRef: string, sessionId: string) => Promise<unknown>;
   replyPermission: (
     requestId: string,
@@ -139,6 +143,18 @@ export function createAgentApiClient(baseUrls: string | string[] = AGENT_API_BAS
       return (payload.sessions ?? []).map(toSessionSummary).filter(isPresent).sort(compareSessionSummaries);
     },
 
+    async getFrontendActionRegistry() {
+      return requestJsonWithFallback<unknown>(candidates, activeBaseUrl, (nextBaseUrl) => { activeBaseUrl = nextBaseUrl; }, "/frontend-action-registry");
+    },
+
+    async submitFrontendActionResult(sessionId, actionId, result) {
+      await requestJsonWithFallback<unknown>(candidates, activeBaseUrl, (nextBaseUrl) => { activeBaseUrl = nextBaseUrl; }, `/frontend-actions/${encodeURIComponent(actionId)}/result`, {
+        method: "POST",
+        headers: { "Content-Type": "application/json", "x-agent-session-id": sessionId },
+        body: JSON.stringify(result)
+      });
+    },
+
     async loadSession(sessionId) {
       const response = await fetchWithFallback(candidates, activeBaseUrl, (nextBaseUrl) => {
         activeBaseUrl = nextBaseUrl;
@@ -540,6 +556,8 @@ function isSessionStreamDataEventType(value: string): value is AgentSessionStrea
     value === "question_request" ||
     value === "question_response" ||
     value === "tool_call" ||
+    value === "frontend_action" ||
+    value === "frontend_action_result" ||
     value === "ui_envelope" ||
     value === "session_title" ||
     value === "done" ||