Centralize configuration into config.yaml

- All settings moved to config.yaml
- configs/load.py reads from config.yaml with env var overrides
- Environment variables still work for backward compatibility
- Added pyyaml to requirements

tests/test_dead_bug_rules.py

@@ -1,9 +1,7 @@
 from __future__ import annotations
 
-from app.exercises.dead_bug.metrics import calculate_metrics
 from app.exercises.dead_bug.rules import detect_diagonal_extension, has_required_visibility, is_ready_position
-from app.exercises.dead_bug.state_machine import DeadBugStateMachine
-from app.exercises.dead_bug.types import DeadBugMetrics, DeadBugPhase, Point
+from app.exercises.dead_bug.types import DeadBugMetrics, Point
 
 
 class TestDeadBugRules: