feat: implement skill - pr-review

[miroslavpojer]

Implement the pr-review skill.

Changes

• skills/pr-review/SKILL.md — new file

Acceptance criteria

• Activates on "review PR", "check this diff", "give feedback on this code"
• Standard review section always appears
• Elevated risk section appears only for auth/security/infra PRs
• API contracts section appears only when contract files are touched
• Dependency section appears only when manifest files are touched
• CI section appears only when workflow files are touched
• IaC section appears only when infra files are touched
• All comments follow What / Why / How to fix format
• Blocker / Important / Nit grouping always present