Add a code review skill (.github/skills/code-review/SKILL.md)

[priya-sundaram-dev]

Fixes: #15167

As requested in #15167 — this is the separate PR for .github/skills/code-review/SKILL.md, expanding the draft into an executable skill.

Credit to @cclauss for the original draft in #15167; this builds on it with the suggestions from my review there.

What it does

Turns the review process into something any reviewer (human or AI) can run the same way every time:

• Mirrors the CONTRIBUTING.md sections explicitly — Before contributing / Is this an algorithm?, Coding Style, Other Requirements for Submissions — as checkable items so nothing is skipped.
• Fixed output shape for the verdict (Is this an algorithm? / duplicate check / coding style / other requirements / verdict) so reviews are comparable and easy to automate later.
• Tone rule: cite the exact rule and suggest the fix rather than a bare 'rejected' — keeps first-time and Hacktoberfest contributors coming back.
• Maps findings to existing labels (require tests, require type hints, require descriptive names, tests are failing, awaiting reviews) so a review naturally produces the label the cleanup scripts key off.

Describe your change:

• Add a skill?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work.
• I know that pull requests will not be merged if they fail the automated tests.