Refactor: enforce canonical messages/output YAML, normalize content_file, and expand fixture-based validation tests

internal/promptdef/testdata/with_default_profile.yaml

@@ -0,0 +1,13 @@
+id: with-default-profile
+version: "1.0.0"
+default_profile: local-default
+inputs:
+  - name: transcript
+    required: true
+messages:
+  - role: user
+    content: "Write output"
+output:
+  format: text
+  validation_mode: none
+  repair_attempts: 0