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

internal/promptdef/testdata/valid_file_backed.yaml

@@ -0,0 +1,14 @@
+id: valid-file-backed
+version: "1.0.0"
+inputs:
+  - name: transcript
+    required: true
+messages:
+  - role: system
+    content: "Return markdown."
+  - role: user
+    content_file: ./messages/user_prompt.tmpl
+output:
+  format: markdown
+  validation_mode: basic
+  repair_attempts: 0