feat: rename skills/skiff to pouch and move ACK state under .pouch

Use ~/.pouch, the pouch CLI, and .pouch.yaml as the SSOT container.
Keep the inner skills/ packages, and store ACK project state in
.pouch/ack instead of docs/ack.

tests/test_ack_delivery.py

@@ -291,7 +291,7 @@ class AckDeliveryValidationTests(unittest.TestCase):
         errors = validate_delivery.validate_tasks_link(contract, tasks)
 
         self.assertIn(
-            "tasks.project.deliveryFile 必须固定为 docs/ack/delivery.yaml",
+            "tasks.project.deliveryFile 必须固定为 .pouch/ack/delivery.yaml",
             errors,
         )
         self.assertIn("delivery.project.name 必须与 tasks.project.name 一致", errors)