[agents-md] Drop `next build` agent-rules gate
Reverts the build-side hard-fail. `next build` no longer checks for
the managed agent-rules block — failing a build mid-pipeline feels
wrong even when an agent is driving, and the dev-server gate already
catches agent-driven projects at the first iteration. `checkAgentRules`
collapses back to `checkAgentRulesForDev` with the `Dev Server exited.`
label hardcoded.
