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/templates/delivery.schema.json

@@ -1,8 +1,8 @@
 {
   "$schema": "http://json-schema.org/draft-07/schema#",
-  "$id": "https://git.yumee.top/laily/skills/skills/ack/templates/delivery.schema.json",
+  "$id": "https://git.yumee.top/laily/pouch/skills/ack/templates/delivery.schema.json",
   "title": "ACK project delivery contract",
-  "description": "docs/ack/delivery.yaml 的权威结构；语义规则由 scripts/validate_delivery.py 补充。",
+  "description": ".pouch/ack/delivery.yaml 的权威结构；语义规则由 scripts/validate_delivery.py 补充。",
   "type": "object",
   "required": [
     "version",