Improve review skill instructions for determining PR context

[paulromano]

Description

This PR addresses two related issues with the reviewing-openmc-code skill:

1. There was no specific instruction to actually read the PR description, which may provide important context about the PR. The first step now is to view the PR (using the gh CLI preferentially) to read it.
2. I was finding when running Claude Code within VS code that it kept comparing against the master branch even though the skill already says to compare against develop. When I asked CC why it did this, it said "the VSCode extension injects a Main branch (you will usually use this for PRs): master hint into every conversation". The updated instructions should avoid this.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[jtramm]

LGTM!