feat: Implement OpenAI proxy with streaming support

- Add main proxy endpoint (api/v1/messages.js)
- Add token validation via GATEWAY_TOKEN
- Add model mapping via MODEL_MAP env var
- Support full SSE streaming in Anthropic format
- Add package.json with openai dependency
- Add vercel.json for routing config
- Update README with Quick Start guide
- Consolidate memory to .claude/memory/

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vercel.json

@@ -0,0 +1,12 @@
+{
+  "rewrites": [
+    {
+      "source": "/v1/messages",
+      "destination": "/api/v1/messages"
+    },
+    {
+      "source": "/v1/:path*",
+      "destination": "/api/v1/:path*"
+    }
+  ]
+}