feat: add GitLab Duo provider extension example

- Delegates to pi-ai's built-in Anthropic/OpenAI streaming
- OAuth support with GitLab's bundled client ID
- Direct access token caching (25 min TTL)
- Models: duo-chat-opus/sonnet/haiku-4-5, duo-chat-gpt-5-1/mini/codex
- No external dependencies beyond pi-ai

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

@@ -0,0 +1,16 @@
+{
+  "name": "pi-extension-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"
+    ]
+  }
+}