Add PostgreSQL kernel review skill - #3

Summary

• Add a reusable Cursor skill for PostgreSQL backend/kernel C code review.
• Cover locking, concurrency, error handling, MemoryContext, ResourceOwner, buffer pins, WAL/recovery, MVCC, hooks, and hot-path performance.
• Include review workflow, finding format, severity guide, testing guidance, GDB snippets, and references to PostgreSQL public/in-tree materials.

Validation

• Ran git diff --check.