feat(FN-4074): add priority option to task creation in engine and extension

Adds task priority support to the task creation flow, spanning the engine runtime (`agent-tools.ts`, `triage.ts`), the CLI extension interface (`extension.ts`), and corresponding documentation and tests. The changeset bumps `@runfusion/fusion` as a minor version.

Fusion-Task-Id: FN-4074

Fusion-Task-Lineage: 726c0ae6-90fb-4c13-ace1-215a2becb2a9

packages/engine/src/__tests__/heartbeat-executor.test.ts

@@ -2603,6 +2603,7 @@ describe("executeHeartbeat", () => {
         description: "Follow-up task",
         dependencies: undefined,
         column: "triage",
+        priority: undefined,
         source: {
           sourceType: "agent_heartbeat",
           sourceAgentId: "agent-001",
@@ -2610,6 +2611,27 @@ describe("executeHeartbeat", () => {
         },
       }, expect.objectContaining({ settings: { autoSummarizeTitles: false } }));
     });
+
+    it("forwards explicit priority when fn_task_create tool is called", async () => {
+      const store = createStoreWithAgentForExec();
+      let capturedCreateTool: any;
+      const mockSession = createMockAgentSession();
+      mockedCreateFnAgent.mockImplementation(async (opts: any) => {
+        capturedCreateTool = opts.customTools[0];
+        return { session: mockSession as any };
+      });
+
+      mockSession.prompt = vi.fn().mockImplementation(async () => {
+        await capturedCreateTool.execute("call-1", { description: "Follow-up task", priority: "high" });
+      });
+
+      const monitor = new HeartbeatMonitor({ store, taskStore: mockTaskStore, rootDir: "/tmp" });
+      await monitor.executeHeartbeat({ agentId: "agent-001", source: "on_demand" });
+
+      expect(mockTaskStore.createTask).toHaveBeenCalledWith(expect.objectContaining({
+        priority: "high",
+      }), expect.any(Object));
+    });
   });
 
   describe("error handling", () => {