feat(bounty): add A2A create_sub_task mechanism

apps/web/src/app/api/mcp/[tool]/route.ts

@@ -4,6 +4,7 @@ import {
   ListOpenTasksRequestSchema,
   ClaimTaskRequestSchema,
   SubmitSolutionRequestSchema,
+  CreateSubTaskRequestSchema,
   TaskStatus,
   JudgeOverallResult
 } from "@agent-bounty/contracts";
@@ -684,6 +685,70 @@ export async function POST(request: NextRequest, props: { params: Promise<{ tool
         });
       }
 
+      case "create_sub_task": {
+        const parsed = CreateSubTaskRequestSchema.parse(body);
+        
+        const subTask = await prisma.$transaction(async (tx) => {
+          const claim = await tx.claim.findUnique({ where: { claim_token: parsed.claim_token } });
+          if (!claim || claim.task_id !== parsed.parent_task_id || claim.status !== TaskStatus.EXECUTING) {
+            throw new Error("Invalid claim token or parent task is not EXECUTING");
+          }
+          if (parsed.reward_amount >= claim.held_amount) {
+             throw new Error("Sub-task reward cannot exceed parent task reward");
+          }
+
+          const newTask = await tx.task.create({
+            data: {
+              title: `[Sub-Task] ${parsed.title}`,
+              description: parsed.description,
+              status: TaskStatus.OPEN,
+              difficulty: "HELLO_WORLD",
+              reward_amount: parsed.reward_amount,
+              reward_currency: claim.held_currency,
+              required_stack: ["A2A", "Agent Sub-Task"],
+              scope_clarity_score: 1.0,
+              parent_task_id: parsed.parent_task_id,
+              created_by_agent: claim.agent_id,
+              acceptance_criteria: parsed.acceptance_criteria as any,
+              is_priority: true, // Sub tasks are high priority to finish the main task faster
+            }
+          });
+          
+          await logAuditEvent(tx, {
+            actorType: "AGENT",
+            actorId: claim.agent_id,
+            action: "CREATE_SUB_TASK",
+            entityType: "TASK",
+            entityId: newTask.id,
+            beforeState: null,
+            afterState: { status: TaskStatus.OPEN, parent: claim.task_id }
+          });
+
+          return newTask;
+        });
+
+        void sendTrafficAlert({
+          level: "info",
+          action: "EXTERNAL_CREATE_SUB_TASK_SUCCESS",
+          surface: "mcp/create_sub_task",
+          actorType: "AGENT",
+          actorId: subTask.created_by_agent!,
+          taskId: subTask.id,
+          message: `A2A 內循環！Agent 發佈了子任務: ${subTask.id}`,
+          metadata: {
+            parent_task_id: parsed.parent_task_id,
+            reward: parsed.reward_amount,
+            payload_summary: summarizeRequestPayload(tool, body),
+          }
+        });
+
+        return NextResponse.json({
+          sub_task_id: subTask.id,
+          status: subTask.status,
+          request_id: requestContext.request_id,
+        });
+      }
+
       case "check_payout_status": {
         const parsed = z.object({ task_id: z.string().uuid() }).parse(body);