Add PR Reviewer mode

[daniel-lxs]

Important

Add pr-reviewer mode with workflow, best practices, and common mistakes for PR review.

• New Mode:
  • Adds pr-reviewer mode in .roomodes for reviewing pull requests with a focus on code quality, structure, and i18n.
• Workflow:
  • Adds 1_workflow.xml with steps for fetching PR info, diff, checking out locally, reviewing, and submitting comments.
• Best Practices:
  • Adds 2_best_practices.xml outlining guidelines for effective PR review.
• Common Mistakes:
  • Adds 3_common_mistakes_to_avoid.xml listing errors to avoid during PR review.

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