Guide GitHub App users through CloudBurn account setup

[dannysteenman]

What

Guide each CloudBurn GitHub App installation through the CloudBurn account setup it needs before pull request cost estimates can run.

Why

An installation that hasn't finished setup should get a clear next step instead of a missing or confusing cost report. Once setup is valid, estimates should resume without extra repository changes.

Scope

• Show where an organization can finish its CloudBurn account setup
• Post a clear setup prompt when cost estimates aren't available yet
• Resume pull request estimates after the account becomes eligible
• Keep existing repositories and installation settings intact during the transition