feat: add interactive skill selector

skiff/skills.py

@@ -42,9 +42,10 @@ def resolve_skill_source(name: str, *, source: str | None = None) -> tuple[Path,
     registry = load_registry()
     if source in (None, "registry") and name in registry:
         path = external_skill_path(name, registry[name])
-        if not path.exists():
+        if not (path / "SKILL.md").is_file():
             raise SystemExit(
-                f"外部 skill {name!r} 尚未 fetch。请先运行: skiff fetch {name}"
+                f"外部 skill {name!r} 尚未 fetch 或 path 中缺少 SKILL.md。"
+                f"请运行: skiff fetch {name}"
             )
         return path, "external"