docs: record the real cause of the assets-only outages

Cloudflare Workers Builds was connected to the repo and deploying on every
push with no build command and root directory '/', so opennextjs-cloudflare
build never ran and the Worker shipped with no script. Builds completed in
zero seconds, and deployment history alternated between good manual Wrangler
deploys and broken git ones.

My 31 Jul diagnosis blamed a bare 'wrangler deploy' and the guard I wrote for
the cutover script could never have caught this, because the deploy did not
run locally.

Both integrations are now disconnected: Cloudflare Workers Builds, and the
Supabase GitHub app on the OLD K&S project which had 'deploy to production'
pointed at a database we migrated off.