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.

nanobot/nanobot.py

@@ -8,6 +8,7 @@ from typing import Any
 
 from nanobot.agent.hook import AgentHook, SDKCaptureHook
 from nanobot.agent.loop import AgentLoop
+from nanobot.providers.image_generation import image_gen_provider_configs
 
 
 @dataclass(slots=True)
@@ -63,12 +64,7 @@ class Nanobot:
 
         loop = AgentLoop.from_config(
             config,
-            image_generation_provider_configs={
-                "openrouter": config.providers.openrouter,
-                "aihubmix": config.providers.aihubmix,
-                "minimax": config.providers.minimax,
-                "gemini": config.providers.gemini,
-            },
+            image_generation_provider_configs=image_gen_provider_configs(config),
         )
         return cls(loop)