feat: add auto-generate models workflow

[nearlyforget]

Description

This PR adds the release automation workflow and aligns the generation script with the current UCP schema structure.

Changes:

• Added .github/workflows/auto-generate.yml to listen for spec-release events.
• Fixed generate_models.sh to match consolidated schema files (e.g., buyer_consent.json instead of create_req/update_req).
• Updated definition keys in generate_models.sh to use dev.ucp.shopping.checkout.
• Automated version bumping in package.json and PR creation.

Motivation:
The previous generation script was pointing to legacy schema files. This PR restores functionality and enables automated updates from the main UCP repository.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• Any dependent changes have been merged and published in downstream modules