docs(skill): add UI support check and data handling gotchas

[antonpk1]

Adds gotchas to the create-mcp-app skill:

1. Checking Host UI Support - hasUiSupport() pattern for conditionally registering UI-enabled tools (assumes feat(server): add getUiCapability helper for capability negotiation #313 landed)

2. Data Handling - content and structuredContent go to model context, _meta does not. Don't put base64 in structuredContent.

3. Describe UI in content - Always include text explaining what widget was rendered so model knows a UI was displayed.

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@modelcontextprotocol/ext-apps

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@modelcontextprotocol/server-basic-react

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@modelcontextprotocol/server-basic-vanillajs

npm i https://pkg.pr.new/modelcontextprotocol/ext-apps/@modelcontextprotocol/server-basic-vanillajs@316

@modelcontextprotocol/server-budget-allocator

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@modelcontextprotocol/server-cohort-heatmap

npm i https://pkg.pr.new/modelcontextprotocol/ext-apps/@modelcontextprotocol/server-cohort-heatmap@316

@modelcontextprotocol/server-customer-segmentation

npm i https://pkg.pr.new/modelcontextprotocol/ext-apps/@modelcontextprotocol/server-customer-segmentation@316

@modelcontextprotocol/server-map

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@modelcontextprotocol/server-pdf

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@modelcontextprotocol/server-scenario-modeler

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@modelcontextprotocol/server-shadertoy

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@modelcontextprotocol/server-sheet-music

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@modelcontextprotocol/server-system-monitor

npm i https://pkg.pr.new/modelcontextprotocol/ext-apps/@modelcontextprotocol/server-system-monitor@316

@modelcontextprotocol/server-threejs

npm i https://pkg.pr.new/modelcontextprotocol/ext-apps/@modelcontextprotocol/server-threejs@316

@modelcontextprotocol/server-transcript

npm i https://pkg.pr.new/modelcontextprotocol/ext-apps/@modelcontextprotocol/server-transcript@316

@modelcontextprotocol/server-video-resource

npm i https://pkg.pr.new/modelcontextprotocol/ext-apps/@modelcontextprotocol/server-video-resource@316

@modelcontextprotocol/server-wiki-explorer

npm i https://pkg.pr.new/modelcontextprotocol/ext-apps/@modelcontextprotocol/server-wiki-explorer@316

commit: e2e8d7d

[jonathanhefner]

I think some of this might be worth housing in TypeDoc examples + docs/patterns.md instead, but we can do that later.

Also, flagging that this depends on #313.

Otherwise, looks good. 👍