winui-design: link to brushes.json from winui-gallery instead of inlining

[nmetulev]

Today the winui-design skill includes brush references inline (see plugins/winui/skills/winui-design/references/brushes-and-icons.md).

We should instead have the CLI / skill reference the canonical brushes.json from the winui-gallery repo so the list stays authoritative and in sync, rather than maintaining a copy in the skill.

Follow-up after the current design skill cleanup (PR #96) lands.

Tasks:

• Validate current inlined brushes cover the full set in brushes.json
• Move to linking/loading brushes.json from the winui-gallery repo
• Remove the inlined brush content from the skill

[nmetulev]

Dupe of #103