feat: make MCP SDK a peer dependency (#168)

* feat: make MCP SDK a peer dependency

- Move @modelcontextprotocol/sdk from dependencies to peerDependencies (^1.20.0)
- Add to devDependencies for local development (^1.24.3)
- Externalize SDK from all build entry points (app, app-bridge, react, server)

Benefits:
- Consumers control their SDK version
- Deduplication when consumer already has SDK
- Follows pattern of react/zod peer deps

* fix: require SDK ^1.24.0 for Zod 4 compatibility

Earlier SDK versions have Zod type incompatibilities that cause build errors.

* chore: update examples to SDK ^1.24.0 to match peer dep

build.bun.ts

@@ -23,13 +23,20 @@ function buildJs(entrypoint: string, opts: Record<string, any> = {}) {
 }
 
 await Promise.all([
-  buildJs("src/app.ts", { outdir: "dist/src" }),
+  buildJs("src/app.ts", {
+    outdir: "dist/src",
+    external: ["@modelcontextprotocol/sdk"],
+  }),
   buildJs("src/app-bridge.ts", {
     outdir: "dist/src",
     external: ["@modelcontextprotocol/sdk"],
   }),
   buildJs("src/react/index.tsx", {
     outdir: "dist/src/react",
-    external: ["react", "react-dom"],
+    external: ["react", "react-dom", "@modelcontextprotocol/sdk"],
+  }),
+  buildJs("src/server/index.ts", {
+    outdir: "dist/src/server",
+    external: ["@modelcontextprotocol/sdk"],
   }),
 ]);