chore: remove webapp config from default appsscript.json

[h13]

Problem

The default appsscript.json includes a webapp section:

"webapp": {
  "access": "ANYONE",
  "executeAs": "USER_ACCESSING"
}

This is only relevant for Web App projects (using doGet/doPost), but many projects from this template are time-driven triggers, custom functions, or other non-Web-App patterns.

Proposed Fix

1. Remove webapp from the default appsscript.json
2. Add a "Web App Configuration" section in README explaining how to add it when needed