Replace hardcoded config and directory scan with YAML config and explicit clip list

- config.py constants -> config/config.yaml (user-editable, git-ignored)
- Questions and defaults now defined in the YAML, including per-question defaults
- ClipSelector no longer scans the data dir; reads a user-provided clips.txt instead
- Removed --daily / --time / --skip-existing-day args
- video_loader now samples frames evenly across the full clip
- pyyaml added as a dependency

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

config/config.example.yaml

@@ -0,0 +1,43 @@
+display_max: 720
+fps_fallback: 25
+max_frames: 100
+
+data_dir: data/filtered_data
+out_dir: data/annotation_results
+clips_file: config/clips.txt
+
+questions:
+  - 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