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

effects_plugins/glitch.py

@@ -5,7 +5,7 @@ from engine.effects.types import EffectConfig, EffectContext, EffectPlugin
 from engine.terminal import C_DIM, DIM, G_DIM, G_LO, RST
 
 
-class GlitchEffect:
+class GlitchEffect(EffectPlugin):
     name = "glitch"
     config = EffectConfig(enabled=True, intensity=1.0)