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>

.gitignore

@@ -11,4 +11,8 @@
 .DS_Store
 
 # Data
-data/**
+data/**
+
+# User-specific config (copy from *.example.* files)
+config/config.yaml
+config/clips.txt