feat(FN-1776): expose AgentStore for SSE event forwarding with ephemeral agent filtering

- Expose getAgentStore from runtime and engine packages
- Add includeSystem filter to getOrgTree API endpoint
- Wire AgentStore into SSE endpoint for event forwarding
- Forward agent events through the SSE pipeline
- Exclude ephemeral agents from org tree API and UI
- Filter ephemeral agents in useAgents hook
- Update AgentsView test to expect includeSystem filter
- Document agent SSE event forwarding architecture

packages/engine/src/runtimes/in-process-runtime.ts

@@ -710,6 +710,14 @@ export class InProcessRuntime
     return this.taskStore;
   }
 
+  /**
+   * Get the AgentStore instance (if initialized).
+   * Returns undefined before start() or if init fails.
+   */
+  getAgentStore(): import("@fusion/core").AgentStore | undefined {
+    return this.agentStore;
+  }
+
   /**
    * Get the project's Scheduler instance.
    * @throws Error if runtime has not been started