Add release bundle skill guidance

[anth-volk]

Fixes #378

Summary

Adds canonical AI-facing guidance for certified release-bundle updates.

The guidance directs agents to use the existing single-country refresh script, process US and UK one at a time, and keep fallback compatibility assertions in country data release manifests instead of manually editing bundled .py manifests.

Dependency

This PR requires PolicyEngine/policyengine-uk-data#424 before the UK fallback path is symmetric with US. The .py guidance references additional_compatible_specifiers for both countries, and UK-data needs #424 to provide that release-manifest utility.

Notes

• Current main already has no bundle_update wrapper or refresh_release_bundles script, so this PR adds guidance rather than deleting wrapper code.
• The fallback path is documented as an unverified publisher assertion: compatible_model_packages is not proof that the model/data setup works.

Validation

• python3 -m py_compile src/policyengine/cli.py
• git diff --check
• Search confirmed no stale bundle_update, refresh_release_bundles, refresh-release-bundles, or --country all references remain.

Could not complete the focused pytest suite locally because collection hung; I stopped the process rather than leaving it running. The changed .py files are documentation/agent guidance only.