feat: add discussion moderation agent

[nearlyforget]

Description

This PR enhances the discussion_moderation_agent by adding a fourth triage trigger: Stalled Q&A detection.

Key Updates:

• Stalled Q&A Trigger: The agent now monitors Q&A discussions. If a discussion is in an answerable category, has no accepted answer, and has remained inactive for
  over one month, it is automatically flagged with the needs-review label.
• GraphQL Query Enhancement: Updated the data fetching logic to include category metadata (isAnswerable) and answer status, providing the agent with the necessary
  context to identify stalled threads.
• Temporal Awareness: Included the current date in the agent's prompt to ensure accurate duration calculations for inactivity.

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.