feat(devgen): add Ollama provider and host configuration

Add support for the Ollama provider in the AI generation pipeline and expose a configurable host address.

New Features:
- Introduce `ollama` as a selectable provider.
- Add `ollama_host` configuration option with default `http://localhost:11434`.

Enhancements:
- Extend provider list in error message to include Ollama.

devgen/ai.py

@@ -24,7 +24,8 @@ def generate_with_ai(
     except ModuleNotFoundError as e:
         raise ImportError(
             f"Provider `{provider}` is not a built-in module: {e}. "
-            f"Available providers: gemini, openai, anthropic, huggingface, openrouter."
+            f"Available providers: gemini, openai, anthropic, huggingface, "
+            f"openrouter, ollama."
         ) from e
     except AttributeError as e:
         raise ImportError(