feat(image-generation): add Gemini provider support

Adds GeminiImageGenerationClient covering both Imagen 4 (:predict) and
Gemini Flash (:generateContent), wires the gemini ProviderConfig through
the SDK, API server, and gateway entry points, and updates the
image-generation docs and skill. Errors from the Gemini endpoints are
logged and surface with the HTTP status and parsed message instead of an
empty string.

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

nanobot/nanobot.py

@@ -67,6 +67,7 @@ class Nanobot:
                 "openrouter": config.providers.openrouter,
                 "aihubmix": config.providers.aihubmix,
                 "minimax": config.providers.minimax,
+                "gemini": config.providers.gemini,
             },
         )
         return cls(loop)