Add X Publisher quality system skill

[yvette-carlisle]

Summary

• add a repo-local x-post-quality-system skill for @decodexspace editorial and visual standards
• route x-post-publisher through the new quality gate before publication/media generation
• define benchmark-derived release/update, changelog, and operator-decision post shapes
• define decodex_signal_card as a unified visual system rather than a fixed reusable image

Validation

• cargo run -p decodex --bin decodex -- radar validate artifacts/social/x

Notes

• This does not publish anything. It makes future automation reject weak/internal candidates and generic/reused media.