vn.py 4.0 split every app (CTA strategy, CTA backtester, ...) into its own
pip-installable package. The vnpy-export skill's generated strategy template
and SKILL.md docs still used the pre-4.0 import path
(`vnpy.app.cta_strategy`), which no longer exists — a strategy file exported
by this skill would fail to import in any current vn.py install.
Verified against a real vn.py 4.0 + vnpy_ctastrategy install: the old import
raises ModuleNotFoundError; the corrected import resolves and the exported
MaCrossStrategy class loads successfully.
Signed-off-by: y85998607 <y85998607@gmail.com>
