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/scroll.py

@@ -11,6 +11,7 @@ from engine import config
 from engine.frame import calculate_scroll_step
 from engine.layers import (
     apply_glitch,
+    process_effects,
     render_firehose,
     render_message_overlay,
     render_ticker_zone,
@@ -18,6 +19,8 @@ from engine.layers import (
 from engine.terminal import CLR
 from engine.viewport import th, tw
 
+USE_EFFECT_CHAIN = True
+
 
 def stream(items, ntfy_poller, mic_monitor):
     """Main render loop with four layers: message, ticker, scroll motion, firehose."""
@@ -42,6 +45,7 @@ def stream(items, ntfy_poller, mic_monitor):
     noise_cache = {}
     scroll_motion_accum = 0.0
     msg_cache = (None, None)
+    frame_number = 0
 
     while True:
         if queued >= config.HEADLINE_LIMIT and not active:
@@ -93,10 +97,24 @@ def stream(items, ntfy_poller, mic_monitor):
         buf.extend(ticker_buf)
 
         mic_excess = mic_monitor.excess
-        buf = apply_glitch(buf, ticker_buf_start, mic_excess, w)
 
-        firehose_buf = render_firehose(items, w, fh, h)
-        buf.extend(firehose_buf)
+        if USE_EFFECT_CHAIN:
+            buf = process_effects(
+                buf,
+                w,
+                h,
+                scroll_cam,
+                ticker_h,
+                mic_excess,
+                grad_offset,
+                frame_number,
+                msg is not None,
+                items,
+            )
+        else:
+            buf = apply_glitch(buf, ticker_buf_start, mic_excess, w)
+            firehose_buf = render_firehose(items, w, fh, h)
+            buf.extend(firehose_buf)
 
         if msg_overlay:
             buf.extend(msg_overlay)
@@ -106,6 +124,7 @@ def stream(items, ntfy_poller, mic_monitor):
 
         elapsed = time.monotonic() - t0
         time.sleep(max(0, config.FRAME_DT - elapsed))
+        frame_number += 1
 
     sys.stdout.write(CLR)
     sys.stdout.flush()