docs: add contributing, governance, code of conduct, and pr template

[nearlyforget]

Description

This pull request move the foundational community health files and contribution standards to organization repository as per the official documentation. It adds the contribution guidelines, governance model, code of conduct, and a standardized pull request template to ensure consistency and transparency for all contributors but it doesn't introduce any changes to existing files in ucp repository.

Category (Required)

• Governance/Contributing: Updates to GOVERNANCE.md or CONTRIBUTING.md
• Documentation: Updates to README, or maturity levels.
• Infrastructure: CI/CD, Linters, or build scripts. (Adding PR Template)

Related Issues

N/A

Checklist

• x] 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.