Rewrite SKILL.md in RFC 2119 style with explicit definitions

Apply MUST/MUST NOT/SHOULD/MAY compliance keywords to every
load-bearing instruction. Remove hedges. Tighten language while
preserving all procedural content. Refresh AGENTS.md from
upstream rules.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: metyatech