Template clean up

[nearlyforget]

Description

This pull request consolidates the repository's issue templates into structured GitHub Issue Forms (YAML) and disables blank issue submissions. By migrating from Markdown to YAML, we ensure that contributors provide essential information (like reproduction steps and environment details) through required fields, improving the quality of incoming reports and streamlining the triage process.

Category (Required)

• Community Health (.github): Updates to the .github repository, including issue/PR templates, automation workflows, or org-level configurations.

Checklist

• I have followed the [Contributing Guide (https://github.com/agentic-commerce/ucp/blob/main/CONTRIBUTING.md).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.