feat(hub): add singleton pattern and subagent spawning

Add global Hub singleton for cross-module access by subagent tools.
Add createSubagent() method to Hub for spawning ephemeral child agents
with isSubagent flag and custom system prompts.

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

src/hub/hub-singleton.ts

@@ -0,0 +1,28 @@
+/**
+ * Global Hub singleton for cross-module access.
+ *
+ * Used by subagent tools and announce flow to interact with the Hub
+ * without threading references through the entire call chain.
+ */
+
+import type { Hub } from "./hub.js";
+
+let _hub: Hub | undefined;
+
+/** Set the global Hub instance. Called once during Hub construction. */
+export function setHub(hub: Hub): void {
+  _hub = hub;
+}
+
+/** Get the global Hub instance. Throws if not yet initialized. */
+export function getHub(): Hub {
+  if (!_hub) {
+    throw new Error("[Hub] Hub singleton not initialized. Ensure Hub is constructed before accessing.");
+  }
+  return _hub;
+}
+
+/** Check if the Hub singleton has been initialized. */
+export function isHubInitialized(): boolean {
+  return _hub !== undefined;
+}

src/hub/hub.ts

@@ -9,7 +9,9 @@ import {
   type ResponseErrorPayload,
 } from "@multica/sdk";
 import { AsyncAgent } from "../agent/async-agent.js";
+import type { AgentOptions } from "../agent/types.js";
 import { getHubId } from "./hub-identity.js";
+import { setHub } from "./hub-singleton.js";
 import { loadAgentRecords, addAgentRecord, removeAgentRecord } from "./agent-store.js";
 import { RpcDispatcher, RpcError } from "./rpc/dispatcher.js";
 import { createGetAgentMessagesHandler } from "./rpc/handlers/get-agent-messages.js";
@@ -48,6 +50,9 @@ export class Hub {
     this.rpc.register("deleteAgent", createDeleteAgentHandler(this));
     this.rpc.register("updateGateway", createUpdateGatewayHandler(this));
 
+    // Register as global singleton for cross-module access (subagent tools, announce flow)
+    setHub(this);
+
     this.client = this.createClient(this.url);
     this.client.connect();
     this.restoreAgents();
@@ -243,6 +248,27 @@ export class Hub {
     }
   }
 
+  /** Create a subagent with specific options (isSubagent, systemPrompt, model) */
+  createSubagent(sessionId: string, options: Omit<AgentOptions, "sessionId"> = {}): AsyncAgent {
+    const existing = this.agents.get(sessionId);
+    if (existing && !existing.closed) {
+      return existing;
+    }
+
+    const agent = new AsyncAgent({
+      ...options,
+      sessionId,
+      isSubagent: true,
+    });
+    this.agents.set(agent.sessionId, agent);
+
+    // Subagents are ephemeral — don't persist to agent store
+    void this.consumeAgent(agent);
+
+    console.log(`[Hub] Subagent created: ${agent.sessionId}`);
+    return agent;
+  }
+
   getAgent(id: string): AsyncAgent | undefined {
     return this.agents.get(id);
   }