refactor: remove /v1/gemini endpoint, replaced by /v1beta routes

- Delete handle_gemini handler (identical to handle_gemini_v1beta)
- Remove /v1/gemini route from router
- Update root handler service name to zerogravity
- Clean all doc references

README.md

@@ -35,7 +35,6 @@ zg start
 | ---------- | --------------------------------- | ------------------------------------ |
 | `POST`     | `/v1/responses`                   | Responses API (sync + streaming)     |
 | `POST`     | `/v1/chat/completions`            | Chat Completions API (OpenAI compat) |
-| `POST`     | `/v1/gemini`                      | Native Gemini API                    |
 | `POST`     | `/v1beta/models/{model}:{action}` | Official Gemini v1beta routes        |
 | `GET/POST` | `/v1/search`                      | Web Search via Google grounding      |
 | `GET`      | `/v1/models`                      | List available models                |