Add review-open-issue skill documenting bug-issue triage practices

[priya-sundaram-dev]

Describe your change

• Documentation change?

Adds .github/skills/review-open-issue/SKILL.md, as requested in #15081 (comment).

It captures, as a repeatable skill, the bug-issue triage practices from the recent bug-label review pass so any triager (human or AI) can run it the same way and end with a maintainer-ready list of Closes #NNNNN lines. Highlights, folding in your notes from #15081:

• Learn-by-doing default — merge an adequate existing contributor PR; only open your own when none is adequate.
• Reproduce before trusting, and say so explicitly when you can't confirm locally rather than guessing.
• Verify each PR's doctest values numerically against a trusted reference; reject out-of-scope "fixes" (e.g. a 1e-15 magic constant hiding a divide-by-zero).
• Markdown autolink hygiene — start every line that references an issue/PR with - /* so GitHub autolinks color them (purple = merged, red = closed, green = open).
• One combined autoclose — since merging a closing PR also closes the issue's other open PRs, fold duplicates into Closes #issue, Closes #dupe, ….

Sits alongside the existing code-review and new-pull-request skills and links back to new-pull-request for the PR mechanics.

Checklist

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