feat: add AI-powered issue triage agent

[nearlyforget]

Description

This PR introduces the issue_triage_agent, an automated assistant powered by Gemini 2.0 Flash designed to streamline issue management through intelligent classification and routing.

Key Capabilities:

• Automated Labeling & Routing: Classifies issues into specific categories (e.g., core-protocol, capability, ucp-schema) and assigns them to the appropriate team or
  committee.
• Template Verification: Validates new issues against the repository's bug report and feature request templates, politely requesting missing information from authors
  via comments.
• Bot & Automation Safety: Automatically detects and skips issues generated by bots (e.g., Dependabot, Renovate) and automated reports (e.g., Weekly Metrics) to
  avoid recursive loops.
• Decoupled Configuration: Extracted business rules (mappings and AI guidelines) into a standalone file, allowing community members to update routing logic without
  touching the core agent code.

Category (Required)

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

Related Issues

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.