refactor(image-generation): introduce provider registry to eliminate manual wiring

Adds ImageGenerationProvider ABC with shared __init__, _http_post(), and
_require_images(). Introduces _IMAGE_GEN_PROVIDERS registry with
register/get/image_gen_provider_configs() helpers.

Four existing providers (OpenRouter, AIHubMix, Gemini, MiniMax) now inherit
from the base class and self-register. Adding a new provider only requires
writing one class + one registration line.

Eliminates if/else chains in the tool dispatch and hardcoded provider config
dicts in commands.py (3 sites) and nanobot.py (1 site). Fixes the agent CLI
command missing image_generation_provider_configs entirely.

Also simplifies test monkeypatch targets to patch the registry lookup.

tests/agent/test_loop_image_generation_media.py

@@ -35,8 +35,8 @@ async def test_generated_image_media_is_attached_to_final_assistant_message(
 ) -> None:
     set_config_path(tmp_path / "config.json")
     monkeypatch.setattr(
-        "nanobot.agent.tools.image_generation.OpenRouterImageGenerationClient",
-        FakeImageClient,
+        "nanobot.agent.tools.image_generation.get_image_gen_provider",
+        lambda name: FakeImageClient if name == "openrouter" else None,
     )
     provider = MagicMock()
     provider.get_default_model.return_value = "test-model"