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>

pyproject.toml

@@ -19,6 +19,7 @@ dependencies = [
      "matplotlib-inline>=0.2.1",
      "pillow>=12.2.0",
      "pyside6>=6.11.0",
+     "pyyaml>=6.0",
 ]
 dynamic = ["version"]