Add doc validation skills for ui-extensions

[rennyG]

Closed in favour of #4085

Adds Claude Code skills for validating documentation updates in this repo. Used by the docs team.

Closes https://github.com/Shopify/temp-project-mover-Archetypically-20260313091522/issues/343

Skills added

• ui-extensions:router — entry point, routes to correct skills by page type
• ui-extensions:style-lint — mechanical text checks (contractions, punctuation, formatting, capitalization, file references)
• ui-extensions:examples — example quality checks covering only what the automated script can't (uniqueness, description pattern, usefulness)
• ui-extensions:components — component description format, best practices, limitations
• ui-extensions:apis — API .doc.ts structure, properties, methods, helper docs
• ui-extensions:type-definitions — JSDoc quality, slots, events, source of truth
• ui-extensions:cross-surface — consistency across admin, checkout, POS
• ui-extensions:link-validation — link verification via MCP tools (run separately)
• ui-extensions:build-verification — docs generation and JSON verification

Design

• All skills invoked manually, not auto-triggered
• Output format: Validation Report (what's good / needs attention / must fix / recommendation)
• Style-lint and examples are split so Claude doesn't context-switch between mechanical scanning and subjective judgment
• Example validation explicitly defers to the existing automated script for syntax/rendering checks