Hint at provider prefix when bare model 401s against OpenAI

A bare model name without a provider prefix routes through the SDK's
default OpenAI provider, so configuring STRIX_LLM=deepseek-v4-pro with
LLM_API_KEY=<deepseek key> sends that key to api.openai.com and
surfaces a confusing "Incorrect API key" error pointing at the OpenAI
dashboard.

When warm-up fails with an OpenAI-shaped error AND the configured
model is still unprefixed after normalize_model_name, append a hint
that points the user at the '<provider>/<model>' form with concrete
examples.

strix/interface/main.py

@@ -5,6 +5,7 @@ Strix Agent Interface
 
 import argparse
 import asyncio
+import contextlib
 import shutil
 import sys
 from datetime import UTC, datetime
@@ -242,6 +243,24 @@ async def warm_up_llm() -> None:
         error_text.append("Please check your configuration and try again.\n", style="white")
         error_text.append(f"\nError: {e}", style="dim white")
 
+        raw_model = ""
+        resolved = ""
+        with contextlib.suppress(Exception):
+            raw_model = (load_settings().llm.model or "").strip()
+            resolved = normalize_model_name(raw_model)
+        err_lc = str(e).lower()
+        unprefixed = bool(resolved) and "/" not in resolved
+        looks_openai = "platform.openai.com" in err_lc or "openai" in err_lc
+        if unprefixed and looks_openai:
+            error_text.append(
+                f"\n\nHint: '{raw_model}' has no provider prefix, so the SDK "
+                f"routed it through OpenAI by default. For non-OpenAI providers "
+                f"use the '<provider>/<model>' form, e.g. "
+                f"'deepseek/deepseek-chat', 'groq/llama-3.1-70b', "
+                f"'anthropic/claude-3-5-sonnet'.",
+                style="yellow",
+            )
+
         panel = Panel(
             error_text,
             title="[bold white]STRIX",