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/tools/test_image_generation_tool.py

@@ -44,8 +44,8 @@ async def test_generate_image_tool_stores_artifact_and_source_images(
     set_config_path(tmp_path / "config.json")
     FakeImageClient.instances = []
     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,
     )
     ref = tmp_path / "ref.png"
     ref.write_bytes(PNG_BYTES)
@@ -98,8 +98,8 @@ async def test_generate_image_tool_selects_aihubmix_provider(
     set_config_path(tmp_path / "config.json")
     FakeImageClient.instances = []
     monkeypatch.setattr(
-        "nanobot.agent.tools.image_generation.AIHubMixImageGenerationClient",
-        FakeImageClient,
+        "nanobot.agent.tools.image_generation.get_image_gen_provider",
+        lambda name: FakeImageClient if name == "aihubmix" else None,
     )
     tool = ImageGenerationTool(
         workspace=tmp_path,