Move questions and optical flow to separate config files; clean up config.example.yaml

- questions: extracted from config.yaml into config/questions.yaml (committed, like optical_flow_config.yaml)
- optical_flow_config_file and questions_config_file are now required fields
- data_dir and out_dir are now required (no defaults)
- filenames: trimmed to input-only in example; output filenames stay as code defaults
- annotator: remove optional guard around optical flow config loading

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/questions.yaml

@@ -0,0 +1,34 @@
+- section: River
+  items:
+    - key: flow
+      label: Flow Regime
+      options: [Turbulent, Laminar, Uncertain]
+      default: Laminar
+    - key: shadows
+      label: Strong Shadows
+      options: [Yes, No, Uncertain]
+      default: No
+    - key: artifacts
+      label: Artifacts on River
+      options: [Yes, No, Uncertain]
+      default: No
+- section: Scene
+  items:
+    - key: lighting
+      label: Lighting
+      options: [Day, Night, Uncertain]
+      default: Day
+    - key: exposure
+      label: Exposure
+      options: [Overexposed, Underexposed, Both, Normal, Uncertain]
+      default: Normal
+- section: Weather
+  items:
+    - key: snowing
+      label: Snowing
+      options: [Yes, No, Uncertain]
+      default: No
+    - key: snow_on_ground
+      label: Snow on Ground
+      options: [Yes, No, Uncertain]
+      default: No