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/tasks.template.yaml

@@ -1,4 +1,4 @@
-# 复制为 docs/ack/tasks.yaml，替换占位符。结构见 templates/tasks.schema.json。
+# 复制为 .pouch/ack/tasks.yaml，替换占位符。结构见 templates/tasks.schema.json。
 version: 1
 updatedAt: "<YYYY-MM-DDTHH:mm:ss+TZ>"
 source: "Coordinator (PM) Agent"
@@ -6,9 +6,9 @@ ackVersion: "<接入时的 ack skill 版本>"
 project:
   name: "<project_name>"
   baseUrl: "<base_url>"
-  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"
   # 可选：飞书 Base Bug 收件箱。只保存 profile 名和资源 ID，绝不保存 App Secret。
   # bugIntake:
   #   provider: "feishu-base"