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/knowledge.template.yaml

@@ -1,4 +1,4 @@
-# 复制为 docs/ack/knowledge.yaml，替换占位符。结构见 templates/knowledge.schema.json。
+# 复制为 .pouch/ack/knowledge.yaml，替换占位符。结构见 templates/knowledge.schema.json。
 # Developer/Test 只能在任务证据中提出 candidate；只有 Coordinator 写入这里。
 version: 1
 updatedAt: "<YYYY-MM-DDTHH:mm:ss+TZ>"