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/cli/commands.py

@@ -643,6 +643,7 @@ def serve(
                 "openrouter": runtime_config.providers.openrouter,
                 "aihubmix": runtime_config.providers.aihubmix,
                 "minimax": runtime_config.providers.minimax,
+                "gemini": runtime_config.providers.gemini,
             },
         )
     except ValueError as exc:
@@ -757,6 +758,7 @@ def _run_gateway(
             "openrouter": config.providers.openrouter,
             "aihubmix": config.providers.aihubmix,
             "minimax": config.providers.minimax,
+            "gemini": config.providers.gemini,
         },
         provider_snapshot_loader=load_provider_snapshot,
         runtime_model_publisher=lambda model, preset: publish_runtime_model_update(