refactor: Make EffectPlugin an abstract base class, update effects to inherit from it, and improve plugin discovery.

effects_plugins/noise.py

@@ -5,7 +5,7 @@ from engine.effects.types import EffectConfig, EffectContext, EffectPlugin
 from engine.terminal import C_DIM, G_DIM, G_LO, RST, W_GHOST
 
 
-class NoiseEffect:
+class NoiseEffect(EffectPlugin):
     name = "noise"
     config = EffectConfig(enabled=True, intensity=0.15)