feat(orchestrator): MS23-P0-005 subagent tree endpoint (#714)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/orchestrator/src/api/agents/agent-tree.service.ts

@@ -0,0 +1,30 @@
+import { Injectable } from "@nestjs/common";
+import { PrismaService } from "../../prisma/prisma.service";
+import { AgentTreeResponseDto } from "./dto/agent-tree-response.dto";
+
+@Injectable()
+export class AgentTreeService {
+  constructor(private readonly prisma: PrismaService) {}
+
+  async getTree(): Promise<AgentTreeResponseDto[]> {
+    const entries = await this.prisma.agentSessionTree.findMany({
+      orderBy: { spawnedAt: "desc" },
+      take: 200,
+    });
+
+    const response: AgentTreeResponseDto[] = [];
+    for (const entry of entries) {
+      response.push({
+        sessionId: entry.sessionId,
+        parentSessionId: entry.parentSessionId ?? null,
+        status: entry.status,
+        agentType: entry.agentType ?? null,
+        taskSource: entry.taskSource ?? null,
+        spawnedAt: entry.spawnedAt.toISOString(),
+        completedAt: entry.completedAt?.toISOString() ?? null,
+      });
+    }
+
+    return response;
+  }
+}