docs(skills): Skill Gallery (web UI) page (new) — BLOCKED on screenshots

[Aaronontheweb]

Part of #95 · Group 2 · NEW skills/gallery.md (Guide/Concept), or a major section on the Skill Server page

Scope: browsing/searching skills, version-history navigation, the resource browser (view SKILL.md/resources), code-copy buttons.

⛔ BLOCKED: needs new browser screenshots — the VHS/TUI capture pipeline can't shoot a web UI. Only config-skills.png / init-07-external-skills.png exist for this area. Deps: #4.