Write a setup troubleshooting page with doctor-based examples

[jnrahme]

Goal

Create one focused troubleshooting page that turns repeated setup pain into something users can self-serve.

Done when

• there is a dedicated troubleshooting doc or README section for common setup failures
• it includes at least one smartassist doctor example
• it links back to the setup-friction issue form when the docs are not enough

Good starting files

• README.md
• docs/
• docs/runbooks/growth-engine.md

Verification

• read the page as if you are brand new to the repo
• make sure the examples match the current CLI surface