feat(build_ai): make the hosted API opt-in with an explicit base_url

[kstonekuan]

HFlowHostedExecution no longer has a default base_url. Constructing it without one raises ValueError: "HFlowHostedExecution needs base_url: contact us to get access to the hosted API". Passing a URL works as before.

• Removes DEFAULT_HFLOW_HOSTED_BASE_URL.
• examples/build_ai_evaluation now defaults to openai-compatible. hflow-hosted reads its URL from BUILD_AI_HOSTED_BASE_URL.
• examples/evaluate_episode.py takes a required --hosted-base-url.
• README, FAQ, the tutorial, the Build AI how-to, and the examples catalog no longer describe the hosted checks as needing no account; they say to contact us for access.

Validated: ruff, ty, the full suite (2523 passed, 8 skipped), and the build_ai_evaluation example suite (45 passed).

[greptile-apps]

Confidence Score: 5/5

[Medium risk] Makes hosted API access require explicit configuration.

The PR appears safe to merge; the previously broken tutorial command is fixed.

Summary

Hosted checks now require an explicit API URL. The Build AI example defaults to an OpenAI-compatible endpoint, and the docs explain how to request hosted access.

• Hosted checks use a URL supplied by the caller.
• The Build AI evaluation example defaults to an OpenAI-compatible endpoint.

_{Reviews (2) · Last reviewed commit: "docs: pass --hosted-base-url in the tuto..." · Reviewed by Greptile}