Docs: Build Your Own Skin

[sampotts]

To both facilitate the skin competition with Contra and to assist developers building their own skins, we should create the following:

• A docs page outlining how to build a skin.
• An installable skill for LLMs to build skins.

[github-actions]

Existing guidance to consolidate and extend:

• Customize skins covers supported overrides and ejecting a packaged skin.
• Architecture distinguishes packaged skins from a UI composed from individual components.
• Skins documents the available presets.

Planning: this is complementary to, not a duplicate of, #2885. Keep it in the v10 roadmap (it already has High priority / Medium effort project fields). Recommend the GA milestone only if the Contra competition requires both deliverables before GA; otherwise schedule it after #2885's CSS/customization guidance decision so the new page and skill describe the supported path accurately. No blocked-by link is implied. Suggested priority: P1 (launch/DX), not P0.