Add review-code-simplicity skill

[shaug]

TL;DR

Adds a local code-simplicity review skill that reduces duplicated policy, unnecessary control flow, and cognitive load without redesigning the solution.

Summary

• Introduces the standalone review-code-simplicity skill and a repository-reuse and cognitive-load rubric.
• Requires concrete, behavior-preserving simplifications with measurable reductions in policies, passes, states, call depth, or setup.
• Rejects wrappers and abstractions that only relocate complexity, while preserving explicit tests for distinct invariants.
• Adds packet and standalone evaluations for shared policy, one-pass processing, pass-through indirection, test setup, security cases, and clean changes.
• Defines the net-cost evidence required before recommending existing or new dependencies and documents the new skill.

Validation

• Repository formatting, linting, skill validation, and all test gates pass.
• Fresh forward evaluations found every seeded simplification and rejected mechanical DRY and style-only feedback.
• An independent final review found no material ticket-scoped issues at the published head.

Tickets

Fixes #8