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>

uv.lock

@@ -1336,6 +1336,7 @@ dependencies = [
     { name = "pandas" },
     { name = "pillow" },
     { name = "pyside6" },
+    { name = "pyyaml" },
 ]
 
 [package.dev-dependencies]
@@ -1353,6 +1354,7 @@ requires-dist = [
     { name = "pandas", specifier = ">=2.3.3" },
     { name = "pillow", specifier = ">=12.2.0" },
     { name = "pyside6", specifier = ">=6.11.0" },
+    { name = "pyyaml", specifier = ">=6.0" },
 ]
 
 [package.metadata.requires-dev]