feat(effects): add plugin architecture for visual effects

- Extract effects as fully decoupled plugins in engine/effects/
- Add EffectConfig, EffectContext dataclasses and EffectPlugin protocol
- Add EffectRegistry for plugin discovery and management
- Add EffectChain for ordered pipeline execution
- Move built-in effects to effects_plugins/ directory
- Add interactive effects config picker during startup
- Add NTFY command handler for /effects commands
- Add tests for effects system (24 new tests)
- Update AGENTS.md with effects plugin documentation
- Add conventional commits section to AGENTS.md

chore: add coverage.xml to .gitignore

effects_plugins/noise.py

@@ -0,0 +1,36 @@
+import random
+
+from engine import config
+from engine.effects.types import EffectConfig, EffectContext, EffectPlugin
+from engine.terminal import C_DIM, G_DIM, G_LO, RST, W_GHOST
+
+
+class NoiseEffect:
+    name = "noise"
+    config = EffectConfig(enabled=True, intensity=0.15)
+
+    def process(self, buf: list[str], ctx: EffectContext) -> list[str]:
+        if not ctx.ticker_height:
+            return buf
+        result = list(buf)
+        intensity = self.config.intensity
+        probability = intensity * 0.15
+
+        for r in range(len(result)):
+            cy = ctx.scroll_cam + r
+            if random.random() < probability:
+                result[r] = self._generate_noise(ctx.terminal_width, cy)
+        return result
+
+    def _generate_noise(self, w: int, cy: int) -> str:
+        d = random.choice([0.15, 0.25, 0.35, 0.12])
+        return "".join(
+            f"{random.choice([G_LO, G_DIM, C_DIM, W_GHOST])}"
+            f"{random.choice(config.GLITCH + config.KATA)}{RST}"
+            if random.random() < d
+            else " "
+            for _ in range(w)
+        )
+
+    def configure(self, cfg: EffectConfig) -> None:
+        self.config = cfg