feat: add Claude backend with automatic Ollama fallback (Claudification Phase 1)

All agents now prefer Claude API when ANTHROPIC_API_KEY is configured,
with automatic fallback to Ollama when offline or unconfigured. New
src/anthropic/ module provides model selection via get_model() factory.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/anthropic/model_selector.py

@@ -0,0 +1,151 @@
+"""
+Model selector for Claude/Ollama backend switching.
+
+Provides automatic model selection with Claude as preferred backend
+and Ollama as offline fallback.
+"""
+
+from typing import Union
+
+from pydantic_ai.models.anthropic import AnthropicModel
+from pydantic_ai.models.openai import OpenAIChatModel
+
+from src.core.config import config
+from src.core.logging_config import get_logger
+
+logger = get_logger(__name__)
+
+# Cached health check result (set once at startup)
+_claude_available: bool | None = None
+
+
+async def check_claude_health() -> bool:
+    """
+    Check if Claude API is reachable and working.
+
+    This should be called once at application startup.
+    The result is cached in `_claude_available`.
+
+    Returns:
+        True if Claude API is accessible, False otherwise.
+    """
+    global _claude_available
+
+    # No API key configured - Claude not available
+    if not config.ANTHROPIC_API_KEY:
+        logger.info(
+            "claude_health_check_skipped",
+            reason="no_api_key",
+        )
+        _claude_available = False
+        return False
+
+    try:
+        from anthropic import AsyncAnthropic
+
+        client = AsyncAnthropic(api_key=config.ANTHROPIC_API_KEY)
+
+        # Minimal API call to verify connectivity
+        # Using a tiny max_tokens to minimize cost
+        await client.messages.create(
+            model=config.ANTHROPIC_MODEL,
+            max_tokens=1,
+            messages=[{"role": "user", "content": "hi"}],
+        )
+
+        _claude_available = True
+        logger.info(
+            "claude_health_check_passed",
+            model=config.ANTHROPIC_MODEL,
+        )
+        return True
+
+    except Exception as e:
+        _claude_available = False
+        logger.warning(
+            "claude_health_check_failed",
+            error=str(e),
+            model=config.ANTHROPIC_MODEL,
+        )
+        return False
+
+
+def is_claude_available() -> bool:
+    """
+    Check if Claude is available (from cached health check result).
+
+    Returns:
+        True if Claude API was reachable at startup, False otherwise.
+
+    Note:
+        Returns False if health check hasn't been run yet.
+        Call `check_claude_health()` at startup first.
+    """
+    return _claude_available is True
+
+
+def get_model(prefer_cloud: bool | None = None) -> Union[AnthropicModel, OpenAIChatModel]:
+    """
+    Get the best available model.
+
+    Returns Claude if available and preferred, otherwise Ollama.
+
+    Args:
+        prefer_cloud: Override config.PREFER_CLOUD_BACKEND for this call.
+                     If None, uses the config value.
+
+    Returns:
+        PydanticAI model instance (AnthropicModel or OpenAIChatModel).
+
+    Example:
+        >>> model = get_model()
+        >>> agent = Agent(model, system_prompt="...")
+    """
+    # Determine preference
+    use_cloud = prefer_cloud if prefer_cloud is not None else config.PREFER_CLOUD_BACKEND
+
+    # Use Claude if available and preferred
+    if use_cloud and is_claude_available():
+        logger.debug(
+            "model_selected",
+            backend="claude",
+            model=config.ANTHROPIC_MODEL,
+        )
+        return AnthropicModel(
+            model_name=config.ANTHROPIC_MODEL,
+            api_key=config.ANTHROPIC_API_KEY,
+        )
+
+    # Fall back to Ollama
+    from src.ollama.provider import get_ollama_provider
+
+    logger.debug(
+        "model_selected",
+        backend="ollama",
+        model=config.OLLAMA_DEFAULT_MODEL,
+        reason="fallback" if use_cloud else "preferred_local",
+    )
+    return OpenAIChatModel(
+        model_name=config.OLLAMA_DEFAULT_MODEL,
+        provider=get_ollama_provider(),
+    )
+
+
+def get_model_info() -> dict:
+    """
+    Get information about the current model configuration.
+
+    Useful for health checks and debugging.
+
+    Returns:
+        Dict with backend, model name, and availability info.
+    """
+    use_cloud = config.PREFER_CLOUD_BACKEND and is_claude_available()
+
+    return {
+        "backend": "claude" if use_cloud else "ollama",
+        "model": config.ANTHROPIC_MODEL if use_cloud else config.OLLAMA_DEFAULT_MODEL,
+        "claude_available": is_claude_available(),
+        "claude_configured": bool(config.ANTHROPIC_API_KEY),
+        "prefer_cloud": config.PREFER_CLOUD_BACKEND,
+    }