feat: add discussion moderation agent workflow

[nearlyforget]

Description

This PR introduces the discussion-moderation.yml workflow to automate the moderation of GitHub discussions. The workflow integrates with an AI-powered moderation agent (hosted in the .github repository) to enforce community standards and identify threads requiring maintainer intervention.

Key Features:

• Trigger Events: Monitors new and edited discussions and comments in real-time.
• Automated Triage: Flags discussions for review based on specific triggers (Direct Mentions, Derailment, CoC violations, and Stalled Q&A).
• Batch Processing: Runs daily at 08:00 UTC to process all open discussions.
• Manual Control: Supports workflow_dispatch to moderate specific discussion numbers on demand.
• Cross-Repo Execution: Securely pulls agent logic and organization-level configuration from the .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

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.