Add "Migrate from OpenAI App" skill to Claude Code plugin

Adds a new skill (`plugins/mcp-apps/skills/migrate-oai-app/SKILL.md`)
that guides developers through migrating OpenAI Apps SDK applications
to the MCP Apps SDK. Includes API mapping tables, CSP configuration
guidance, and a "Before Finishing" checklist.

Also fixes the `registerAppResource()` JSDoc example in `src/server/index.ts`
to show CSP configuration in the correct location (on the content item,
not the resource config).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/mcp-apps/README.md

@@ -12,9 +12,17 @@ Install via Claude Code:
 
 ## Usage
 
-Invoke the skill by asking Claude Code to:
+Invoke skills by asking Claude Code to:
+
+### Create MCP App
 
 - "Create an MCP App"
 - "Add a UI to an MCP tool"
 - "Build an interactive MCP widget"
 - "Scaffold an MCP App"
+
+### Migrate from OpenAI
+
+- "Migrate from OpenAI Apps SDK"
+- "Convert OpenAI App to MCP"
+- "Port from window.openai"