Pin and expose the shared frontend version used by agent templates, and enforce compatibility

[mdbecker]

agent-openai-agents-sdk depends on e2e-chatbot-app-next, but that dependency is currently implicit in the startup/build flow.

This becomes painful once the template is forked or filtered into its own repo: the app still depends on frontend code that is not actually present or versioned in the downstream project.

Could the frontend dependency be made more explicit and reliable? For example:

• record the frontend repo + ref/commit in config or env
• document the expected frontend/backend interface at a high level
• fail fast if the fetched frontend is not a supported version

That would make builds reproducible, make upgrades safer, and make it much easier to understand/debug frontend/backend version drift.