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/project-engine.ts

@@ -263,6 +263,11 @@ export class ProjectEngine {
     return this.runtime.getTaskStore();
   }
 
+  /** Get the AgentStore (if initialized). Returns undefined before start(). */
+  getAgentStore(): import("@fusion/core").AgentStore | undefined {
+    return this.runtime.getAgentStore();
+  }
+
   /** Get the HeartbeatMonitor (if initialized). */
   getHeartbeatMonitor() {
     return this.runtime.getHeartbeatMonitor();