Add review-solution-simplicity skill

[shaug]

TL;DR

Adds a whole-solution review skill that removes requirement-free machinery while protecting complexity needed for correctness and operations.

Summary

• Introduces the standalone review-solution-simplicity skill and a mechanism-to-requirement comparison rubric.
• Requires every proposed simplification to be concrete, requirement-complete, and explicit about preserved failure semantics.
• Protects justified concurrency, idempotency, security, migration, compatibility, rollout, and recovery behavior.
• Adds packet and standalone evaluations for speculative frameworks, pre-release backfills, necessary concurrency controls, and missing design decisions.
• Aligns explicit ticket-scope violations with the shared blocking-severity rule and documents the new skill.

Validation

• Repository formatting, linting, skill validation, and all test gates pass.
• Fresh forward evaluations found unsupported machinery, preserved necessary complexity, and returned blocked when a smaller design required missing product decisions.
• An independent final review found no material ticket-scoped issues at the published head.

Tickets

Fixes #7