Add PR review guidelines

[bhouston]

This PR addresses issue #350 by adding PR review guidelines.

Changes

• Added .mycoder/PR_REVIEW.md with comprehensive PR review guidelines
• The guidelines cover:
  • Issue alignment
  • Code quality
  • Function and component design
  • Testing
  • Documentation
  • Performance considerations
  • Security considerations
  • Accessibility
  • Browser/environment compatibility
  • Follow-up review guidelines

Note on PR Review Workflow

I also created a PR review GitHub Action workflow file (.github/workflows/pr-review.yml) based on the existing issue-comment.yml template, but I was unable to include it in this PR due to permission limitations:

! [remote rejected] feature/pr-review-action -> feature/pr-review-action (refusing to allow a GitHub App to create or update workflow `.github/workflows/pr-review.yml` without `workflows` permission)

The workflow would:

1. Trigger when a PR is opened or updated with new commits
2. Extract the linked issue from the PR
3. Get previous review comments for context
4. Run MyCoder to review the PR according to the guidelines in .mycoder/PR_REVIEW.md

The workflow file needs to be added manually by someone with appropriate permissions. I can provide the content of the workflow file if needed.

Fixes #350