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-lock.json

@@ -0,0 +1,24 @@
+{
+  "name": "pi-extension-custom-provider",
+  "version": "1.0.0",
+  "lockfileVersion": 3,
+  "requires": true,
+  "packages": {
+    "": {
+      "name": "pi-extension-custom-provider",
+      "version": "1.0.0",
+      "dependencies": {
+        "@anthropic-ai/sdk": "^0.52.0"
+      }
+    },
+    "node_modules/@anthropic-ai/sdk": {
+      "version": "0.52.0",
+      "resolved": "https://registry.npmjs.org/@anthropic-ai/sdk/-/sdk-0.52.0.tgz",
+      "integrity": "sha512-d4c+fg+xy9e46c8+YnrrgIQR45CZlAi7PwdzIfDXDM6ACxEZli1/fxhURsq30ZpMZy6LvSkr41jGq5aF5TD7rQ==",
+      "license": "MIT",
+      "bin": {
+        "anthropic-ai-sdk": "bin/cli"
+      }
+    }
+  }
+}