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/core/config.py

@@ -64,7 +64,21 @@ class Config(BaseSettings):
     API_PORT: int = Field(default=8000, description="API port")
     API_PREFIX: str = Field(default="/v1", description="API route prefix")
     
-    # Ollama Configuration
+    # Anthropic Configuration (Claude - preferred backend)
+    ANTHROPIC_API_KEY: str | None = Field(
+        default=None,
+        description="Anthropic API key for Claude access"
+    )
+    ANTHROPIC_MODEL: str = Field(
+        default="claude-sonnet-4-20250514",
+        description="Claude model to use"
+    )
+    PREFER_CLOUD_BACKEND: bool = Field(
+        default=True,
+        description="Prefer Claude over Ollama when available"
+    )
+
+    # Ollama Configuration (local fallback)
     OLLAMA_HOST: HttpUrl = Field(
         default="http://localhost:11434",
         description="Ollama server URL"