docs: add PR review skill for Review Agent

[9pace]

Adds skills/pr-review, a general PR review skill that review agents load.

• SKILL.md: scope, review process, harm-based severity (BLOCKING / RECOMMENDED / OPTIONAL), what not to flag, and the structured finding format.
• references/review-rules.md: 17 rules covering compatibility, architecture, error handling and UX, testing, code quality, verification, and PR scope. Where possible, each rule cites the relevant section of AGENTS.md or another doc in this repo.

The skill favors precision: it only flags concrete defects that the PR itself introduces, and a review with zero findings is valid. It adds documentation only, with no code or build changes.

Checklist

• This change contains a major version upgrade for a dependency and I confirm all breaking changes are addressed
  • Release notes for the new version:

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.64%. Comparing base (c5d3958) to head (f14d1ab).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1996      +/-   ##
==========================================
- Coverage   91.67%   91.64%   -0.04%     
==========================================
  Files          82       82              
  Lines       13039    13039              
  Branches     1826     1824       -2     
==========================================
- Hits        11954    11950       -4     
- Misses       1047     1051       +4     
  Partials       38       38              

Flag	Coverage Δ
suite.unit	91.64% <ø> (-0.04%)	⬇️

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