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

AGENTS.md

@@ -108,3 +108,78 @@ The project uses pytest with strict marker enforcement. Test configuration is in
 - **eventbus.py** provides thread-safe event publishing for decoupled communication
 - **controller.py** coordinates ntfy/mic monitoring
 - The render pipeline: fetch → render → effects → scroll → terminal output
+
+## Effects Plugin System
+
+The effects system is implemented as a plugin architecture in `engine/effects/`.
+
+### Core Components
+
+| Module | Purpose |
+|--------|---------|
+| `effects/types.py` | `EffectConfig`, `EffectContext` dataclasses and `EffectPlugin` protocol |
+| `effects/registry.py` | Plugin discovery and management (`EffectRegistry`) |
+| `effects/chain.py` | Ordered pipeline execution (`EffectChain`) |
+| `effects_plugins/*.py` | Externalized effect plugins |
+
+### Creating a New Effect
+
+Create a file in `effects_plugins/` with a class ending in `Effect`:
+
+```python
+from engine.effects.types import EffectConfig, EffectContext
+
+class MyEffect:
+    name = "myeffect"
+    config = EffectConfig(enabled=True, intensity=1.0)
+    
+    def process(self, buf: list[str], ctx: EffectContext) -> list[str]:
+        # Process buffer and return modified buffer
+        return buf
+    
+    def configure(self, config: EffectConfig) -> None:
+        self.config = config
+```
+
+### NTFY Commands
+
+Send messages to the ntfy topic to control effects:
+
+```
+/effects list
+/effects noise on
+/effects noise off
+/effects noise intensity 0.5
+/effects reorder noise,glitch,fade,firehose
+```
+
+## Conventional Commits
+
+Commit messages follow the [Conventional Commits](https://www.conventionalcommits.org/) specification:
+
+```
+<type>(<scope>): <description>
+
+[optional body]
+
+[optional footer(s)]
+```
+
+### Types
+
+- `feat`: A new feature
+- `fix`: A bug fix
+- `docs`: Documentation only changes
+- `style`: Changes that don't affect code meaning (formatting)
+- `refactor`: Code change that neither fixes a bug nor adds a feature
+- `test`: Adding or updating tests
+- `chore`: Changes to build process, dependencies, etc.
+
+### Examples
+
+```
+feat(effects): add plugin architecture for visual effects
+fix(layers): resolve glitch effect not applying on empty buffer
+docs(AGENTS.md): add effects plugin system documentation
+test(effects): add tests for EffectChain pipeline ordering
+```