Add postinstall script to scaffolded package.json

[tobiu]

Update tasks/createPackageJson.mjs to include a postinstall script in the generated package.json.

Reasoning:
The AI MCP servers (defined in ai:mcp-server-* scripts) rely on the neo.mjs package being fully installed. A postinstall script ensures that npm install on the generated workspace correctly sets up the neo.mjs dependency, guaranteeing the servers are executable.