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.

skills/ack/examples/tasks.example.yaml

@@ -3,13 +3,13 @@
 version: 1
 updatedAt: "2026-07-06T09:40:00+08:00"
 source: "Coordinator (PM) Agent"
-ackVersion: "0.18.0"
+ackVersion: "0.19.0"
 project:
   name: "notes-web"
   baseUrl: "http://localhost:5173"
-  overlayFile: "docs/ack/project.md"
-  knowledgeFile: "docs/ack/knowledge.yaml"
-  deliveryFile: "docs/ack/delivery.yaml"
+  overlayFile: ".pouch/ack/project.md"
+  knowledgeFile: ".pouch/ack/knowledge.yaml"
+  deliveryFile: ".pouch/ack/delivery.yaml"
   orchestration:
     profileVersion: 1
     mode: "manual"