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

internal/promptdef/testdata/valid_inline.yaml

@@ -0,0 +1,18 @@
+id: valid-inline
+version: "1.0.0"
+inputs:
+  - name: transcript
+    required: true
+    content_type: text/markdown
+    description: Transcript content
+messages:
+  - role: system
+    content: "You are concise."
+  - role: user
+    content: |
+      Summarize:
+      {{input "transcript"}}
+output:
+  format: markdown
+  validation_mode: basic
+  repair_attempts: 0