refactor: rename custom provider examples

- custom-provider -> custom-provider-anthropic
- gitlab-duo -> custom-provider-gitlab-duo
- Update READMEs with custom provider section

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

@@ -0,0 +1,16 @@
+{
+  "name": "pi-extension-custom-provider-gitlab-duo",
+  "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"
+    ]
+  }
+}