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>

src/river_annotation_tool/annotator.py

@@ -43,11 +43,7 @@ class Annotator(QMainWindow):
         self.fs = fs
         self.out_dir = config.out_dir
         self.extras = extras
-        self.of_cfg = (
-            load_optical_flow_config(Path(config.optical_flow_config_file))
-            if config.optical_flow_config_file
-            else None
-        )
+        self.of_cfg = load_optical_flow_config(Path(config.optical_flow_config_file))
 
         self.selector = ClipSelector(
             data_dir=config.data_dir,
@@ -171,7 +167,7 @@ class Annotator(QMainWindow):
         btn_redo = QPushButton("Redo")
         btn_load_prev_mask = QPushButton("Load Prev Mask")
         btn_auto_segment = QPushButton("Auto Segment")
-        btn_auto_segment.setEnabled(self.of_cfg is not None and self.of_cfg.enabled)
+        btn_auto_segment.setEnabled(self.of_cfg.enabled)
 
         row1 = QHBoxLayout()
         for b in [