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

engine/effects/types.py

@@ -0,0 +1,39 @@
+from dataclasses import dataclass, field
+from typing import Any
+
+
+@dataclass
+class EffectContext:
+    terminal_width: int
+    terminal_height: int
+    scroll_cam: int
+    ticker_height: int
+    mic_excess: float
+    grad_offset: float
+    frame_number: int
+    has_message: bool
+    items: list = field(default_factory=list)
+
+
+@dataclass
+class EffectConfig:
+    enabled: bool = True
+    intensity: float = 1.0
+    params: dict[str, Any] = field(default_factory=dict)
+
+
+class EffectPlugin:
+    name: str
+    config: EffectConfig
+
+    def process(self, buf: list[str], ctx: EffectContext) -> list[str]:
+        raise NotImplementedError
+
+    def configure(self, config: EffectConfig) -> None:
+        raise NotImplementedError
+
+
+@dataclass
+class PipelineConfig:
+    order: list[str] = field(default_factory=list)
+    effects: dict[str, EffectConfig] = field(default_factory=dict)