Add AI-powered PR review workflow via docker/cagent-action

Introduce a GitHub Actions workflow that triggers an AI-powered code
  review on pull requests using docker/cagent-action's reusable workflow.

  - Restrict comment-triggered runs to OWNER/MEMBER/COLLABORATOR
  - Gate on github.repository to prevent execution on forks
  - Filter out draft PRs and bot actors
  - Only trigger on PR comments, not plain issue comments
  - Serialize reviews per PR via concurrency group

Signed-off-by: Guillaume Lours <glours@users.noreply.github.com>