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.

bin/pouch

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-# 跟随软链，确保从 ~/.local/bin/skiff 调用时仍能找到仓库根目录
+# 跟随软链，确保从 ~/.local/bin/pouch 调用时仍能找到仓库根目录
 SCRIPT="${BASH_SOURCE[0]}"
 while [ -L "$SCRIPT" ]; do
   link_dir="$(cd "$(dirname "$SCRIPT")" && pwd)"
@@ -11,4 +11,4 @@ done
 REPO_ROOT="$(cd "$(dirname "$SCRIPT")/.." && pwd)"
 
 export PYTHONPATH="${REPO_ROOT}${PYTHONPATH:+:$PYTHONPATH}"
-exec python3 -m skiff "$@"
+exec python3 -m pouch "$@"