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/builder/README.md

@@ -47,5 +47,5 @@ DOCKER_REGISTRY=registry.example.com
 | `scripts/publish_docker.sh` | buildx 构建 + 推送镜像，远端 digest 验证 |
 | `scripts/verify_deb.sh` | 核对包元数据、内容与 SHA-256 |
 
-环境变量契约、脚本解析顺序（`$BUILDER_SKILL_DIR` → `~/.skills/skills/builder/scripts/`）、
+环境变量契约、脚本解析顺序（`$BUILDER_SKILL_DIR` → `~/.pouch/skills/builder/scripts/`）、
 脏工作树策略等完整规则见 contract.md。