feat(FN-984): add agent hierarchy tree view with parent-child relationships

- Add GET /agents/:id/children API endpoint to fetch child agents of a parent
- Create fetchAgentChildren API function with tests in dashboard client
- Implement useAgentHierarchy hook with recursive tree loading and expand/collapse
- Add tree view mode to AgentsView with hierarchical agent display
- Enhance AgentDetailView with parent info and expandable children section
- Add tree view CSS styles for connection lines and expand/collapse indicators

packages/dashboard/app/api.ts

@@ -1834,6 +1834,15 @@ export function fetchAgentStats(projectId?: string): Promise<AgentStats> {
   return api<AgentStats>(withProjectId("/agents/stats", projectId));
 }
 
+/** Fetch child agents that report to a given parent agent */
+export function fetchAgentChildren(agentId: string, projectId?: string): Promise<Agent[]> {
+  return api<Agent[]>(withProjectId(`/agents/${encodeURIComponent(agentId)}/children`, projectId)).catch((err: Error) => {
+    // Return empty array for 404 (agent may have been deleted)
+    if (err.message.includes("not found")) return [];
+    throw err;
+  });
+}
+
 // ── Agent Generation API ────────────────────────────────────────────────────
 
 /** Generated agent specification returned by the AI */