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/core/src/agent-store.ts

@@ -1500,10 +1500,11 @@ export class AgentStore extends EventEmitter {
 
   /**
    * Build the recursive org tree for all agents.
+   * @param filter - Optional filter for listing agents
    * @returns Root nodes with nested children
    */
-  async getOrgTree(): Promise<OrgTreeNode[]> {
-    const agents = await this.listAgents();
+  async getOrgTree(filter?: { includeSystem?: boolean }): Promise<OrgTreeNode[]> {
+    const agents = await this.listAgents(filter);
     if (agents.length === 0) {
       return [];
     }