feat: add issue triage workflow

[nearlyforget]

Description

This PR introduces the issue-triage.yml workflow, which acts as the integration point for the automated issue triaging system. This workflow executes the AI-powered agent hosted in the .github repository to ensure every new issue is categorized, labeled, and assigned to the correct technical council or maintainers group.

Workflow:

• Event-Driven: Runs instantly when an issue is opened or edited.
• Batch Processing: Runs on a daily schedule (07:00 UTC) to catch any issues that might have been missed or updated without triggering an event.
• Cross-Repo Integration: Securely pulls the latest triaging logic and issue templates from the organization's .github repository.

⚠️ Important Requirement:
This workflow depends on the GOOGLE_API_KEY secret to communicate with the Gemini 2.0 Flash model. We are currently waiting for this API key to be provisioned; once available, it must be added to the repository secrets for the agent to become fully operational.

Category (Required)

• Core Protocol
• Governance/Contributing
• Capability
• Documentation
• Infrastructure: CI/CD, Linters, or build scripts.
• Maintenance
• SDK
• Samples / Conformance

Related Issues

• Depends on the agent implementation in: Universal-Commerce-Protocol/.github#PR_NUMBER

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.