refactor(core-ai): simplify model list to only Tatlock agent

Removes the "simple" fallback model and renames "pydantic" to "Tatlock"
to match the agent's British butler persona.

Changes:
- /models endpoint now returns only "Tatlock" model
- Removed "simple" model from advertised models
- Updated default model name from "pydantic" to "Tatlock"
- Updated health endpoint to show "Tatlock" agent status
- Added description: "PydanticAI agent with full tool support - your British butler assistant"

Benefits:
- Clearer model naming that matches agent persona
- Simplified model selection in Open WebUI
- Eliminates confusion between pydantic/simple models
- Consistent branding with Tatlock character

Open WebUI will now show only "Tatlock" as an available model, which uses
the full PydanticAI agent with tool calling capabilities.

Tested:
✅ /models endpoint returns only Tatlock
✅ Health check shows Tatlock as default agent
✅ Chat completions work with model="Tatlock"
✅ Tatlock persona responds correctly

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

services/core-ai/main.py

@@ -46,7 +46,7 @@ async def chat_completions(request):
 
         # Extract relevant fields from the request
         messages = data.get("messages")
-        model = data.get("model", "pydantic")
+        model = data.get("model", "Tatlock")
         stream = data.get("stream", False)
         conversation_id = data.get("conversation_id")
         enable_tools = data.get("enable_tools", True)
@@ -249,16 +249,11 @@ async def list_models(request):
         "object": "list",
         "data": [
             {
-                "id": "pydantic",
+                "id": "Tatlock",
                 "object": "model",
                 "created": int(time.time()),
-                "owned_by": "core-ai"
-            },
-            {
-                "id": "simple",
-                "object": "model",
-                "created": int(time.time()),
-                "owned_by": "core-ai"
+                "owned_by": "core-ai",
+                "description": "PydanticAI agent with full tool support - your British butler assistant"
             }
         ]
     })
@@ -295,10 +290,9 @@ async def health_check(request):
         "status": "ok",
         "service": "core-ai",
         "agents": {
-            "simple": True,
-            "pydantic": PYDANTIC_AI_AVAILABLE
+            "Tatlock": PYDANTIC_AI_AVAILABLE
         },
-        "default_agent": "pydantic" if PYDANTIC_AI_AVAILABLE else "simple",
+        "default_agent": "Tatlock" if PYDANTIC_AI_AVAILABLE else None,
         "tools_count": len(get_all_tools())
     })