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

internal/promptdef/testdata/duplicate_input_names.yaml

@@ -0,0 +1,14 @@
+id: duplicate-input-names
+version: "1.0.0"
+inputs:
+  - name: transcript
+    required: true
+  - name: transcript
+    required: false
+messages:
+  - role: user
+    content: "Hi"
+output:
+  format: text
+  validation_mode: none
+  repair_attempts: 0