Document single-country release-bundle fallback workflow

[anth-volk]

Summary

Add canonical AI-facing release-bundle guidance for updating certified policyengine.py bundles using the existing single-country refresh script and country-data release-manifest compatibility fallback.

Motivation

Bundle updates should not rely on an AI manually editing bundled .py manifests. Agents need explicit guidance to:

• use scripts/refresh_release_bundle.py for one country at a time,
• process US and UK sequentially when both are requested,
• keep fallback compatibility assertions in country data release manifests through compatible_model_packages, and
• tell users that fallback compatibility is an unverified publisher assertion rather than proof that the setup works.

Dependency

Requires PolicyEngine/policyengine-uk-data#424 so UK data has the same additional_compatible_specifiers release-manifest utility that US data already has.

Acceptance criteria

• Release-bundle guidance points agents to the existing single-country script, not a bundle wrapper or all-country command.
• Fallback instructions require country-data manifest tooling and prohibit manual .py bundle manifest edits.
• The guidance explicitly warns that compatible_model_packages fallback is not validated or verified.