docs: rewrite README and expand package keywords for discoverability

- Lead with value proposition, ASCII diagram, and feature table
- Quickstart reduced to 4 steps; works in under 60 seconds
- SDK examples for OpenAI, Anthropic, Gemini (JS+Python), LangChain
- UI integration guides: Open WebUI, Chatbox, Continue, Zed
- Reference section kept concise; full prose docs moved inline
- package.json: sharper description, 20 keywords covering all search terms
  (openai-compatible, anthropic, gemini, ollama, langchain, open-webui,
   llm-proxy, ai-gateway, local-llm, github-copilot, model-router, …)

Author: KochC