Refactor: split prompt definition from execution settings and migrate run contracts to prompt_* + execution_target

internal/profile/testdata/negative_timeout.yaml

@@ -1,10 +1,11 @@
 id: negative-timeout
 version: "1.0.0"
+inputs:
+  - name: transcript
+    required: true
 templates:
   - role: user
     content: "Say hi"
-model_defaults:
-  timeout_seconds: -1
 output_format: text
 validation:
   validation_mode: none