refactor: convert custom-provider example to package with deps

- Move custom-provider.ts to custom-provider/index.ts
- Add package.json with @anthropic-ai/sdk dependency
- Extension now requires npm install before use

packages/coding-agent/examples/extensions/custom-provider/package.json

@@ -0,0 +1,19 @@
+{
+  "name": "pi-extension-custom-provider",
+  "private": true,
+  "version": "1.0.0",
+  "type": "module",
+  "scripts": {
+    "clean": "echo 'nothing to clean'",
+    "build": "echo 'nothing to build'",
+    "check": "echo 'nothing to check'"
+  },
+  "pi": {
+    "extensions": [
+      "./index.ts"
+    ]
+  },
+  "dependencies": {
+    "@anthropic-ai/sdk": "^0.52.0"
+  }
+}