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>

.claude/memory/MEMORY.md

@@ -0,0 +1,4 @@
+# Project Memory Index
+
+- [Memory Saving Rules](memory_saving_rules.md) — How to save memories in this project
+- [Implementation Plan](implementation_plan.md) — Architecture and design decisions for Claude Central Gateway