fix(config): accept 'model' key as alias for 'default' in model config (#3603)

Users intuitively write model: { model: my-model } instead of
model: { default: my-model } and it silently falls back to the
hardcoded default. Now both spellings work across all three config
consumers: runtime_provider, CLI, and gateway.

Co-authored-by: ygd58 <ygd58@users.noreply.github.com>

cli-config.yaml.example

@@ -7,6 +7,7 @@
 # =============================================================================
 model:
   # Default model to use (can be overridden with --model flag)
+  # Both "default" and "model" work as the key name here.
   default: "anthropic/claude-opus-4.6"
   
   # Inference provider selection: