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>

src/server/index.ts

@@ -243,15 +243,22 @@ export function registerAppTool<
  * ```typescript
  * registerAppResource(server, "Music Player", "ui://music/player.html", {
  *   description: "Audio player with external soundfonts",
- *   _meta: {
- *     ui: {
- *       csp: {
- *         connectDomains: ["https://api.example.com"],  // For fetch/WebSocket
- *         resourceDomains: ["https://cdn.example.com"], // For scripts/styles/images
+ * }, async () => ({
+ *   contents: [{
+ *     uri: "ui://music/player.html",
+ *     mimeType: RESOURCE_MIME_TYPE,
+ *     text: PLAYER_HTML,
+ *     // CSP must be on the content item, not the resource config
+ *     _meta: {
+ *       ui: {
+ *         csp: {
+ *           connectDomains: ["https://api.example.com"],  // For fetch/WebSocket
+ *           resourceDomains: ["https://cdn.example.com"], // For scripts/styles/images
+ *         },
  *       },
  *     },
- *   },
- * }, readCallback);
+ *   }],
+ * }));
  * ```
  *
  * @see {@link registerAppTool} to register tools that reference this resource