fix(im): pin weixin sidecar sdk install

[howold-lab]

Summary

• add a dedicated scripts/im package manifest and lockfile for the weixin sidecar
• pin the sidecar install to weixin-agent-sdk@0.4.0 and update install guidance
• point weixin sidecar import failures at scripts/im npm install instead of an unconstrained global install

Verification

• npm install (in scripts/im)
• node -e "import('weixin-agent-sdk').then((m)=>console.log(typeof m.isLoggedIn, typeof m.start, typeof m.login))"