ci: add AI Review Bot and Gate workflows

[qrtp]

Summary

• Add AI Review Bot workflow (ai-review-bot.yml) — triggers AI code review on label or review request
• Add AI Review Gate workflow (ai-review-gate.yml) — enforces merge gate requiring AI or human approval

Both workflows call centralized reusable workflows from agent-plugins so they stay up to date automatically.

Prerequisites

The following org-level secrets and variables must be configured for this repo:

• Secrets: UD_PAT, ANTHROPIC_API_KEY (and optionally CODEX_API_KEY for multi-agent reviews)
• Variables: AI_REVIEW_BOT_USER

Test plan

• Verify workflows appear in the Actions tab after merge
• Open a test PR and confirm the AI Review Gate runs on PR open
• Add ai-review-requested label to a PR and confirm the AI Review Bot triggers

🤖 Generated with Claude Code

[qrtp]

Closing: public repo — AI review workflows use secrets that aren't available to public repo PR workflows.