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.

tests/test_ack_launch_worker.py

@@ -435,7 +435,7 @@ class PlanTests(unittest.TestCase):
     def test_authoritative_board_is_derived_from_project_root_without_repo_path(self) -> None:
         with tempfile.TemporaryDirectory() as temporary:
             project = Path(temporary).resolve()
-            ack_dir = project / "docs" / "ack"
+            ack_dir = project / ".pouch" / "ack"
             ack_dir.mkdir(parents=True)
             task_board = board(project)
             del task_board["project"]["repoPath"]
@@ -454,7 +454,7 @@ class PlanTests(unittest.TestCase):
     def test_authoritative_board_ignores_legacy_repo_path(self) -> None:
         with tempfile.TemporaryDirectory() as temporary:
             project = Path(temporary).resolve()
-            ack_dir = project / "docs" / "ack"
+            ack_dir = project / ".pouch" / "ack"
             ack_dir.mkdir(parents=True)
             task_board = board(project)
             task_board["project"]["repoPath"] = "/legacy/other-worktree"