fix: restore Groq default model with bundled crush provider

openrouter/qwen/qwen3.6-27b is not resolvable in Crush; the qwen model
only exists on Groq. Re-apply the 735229b provider fix: default to
groq/openai/gpt-oss-120b (verified live on Groq /openai/v1/models) via
the bundled crush-provider config, applied through CRUSH_GLOBAL_CONFIG
only when the default model is in use.

crush-provider/crush.json

@@ -0,0 +1,18 @@
+{
+  "$schema": "https://charm.land/crush.json",
+  "providers": {
+    "groq": {
+      "type": "openai-compat",
+      "base_url": "https://api.groq.com/openai/v1",
+      "api_key": "$GROQ_API_KEY",
+      "models": [
+        {
+          "id": "openai/gpt-oss-120b",
+          "name": "GPT-OSS 120B on Groq",
+          "context_window": 131072,
+          "default_max_tokens": 8192
+        }
+      ]
+    }
+  }
+}