feat: add SixelDisplay backend for terminal graphics

- Implement pure Python Sixel encoder (no C dependency)
- Add SixelDisplay class to display.py with ANSI parsing
- Update controller._get_display() to handle sixel mode
- Add --display sixel CLI flag
- Add mise run-sixel task
- Update docs with display modes

pyproject.toml

@@ -33,6 +33,9 @@ mic = [
 websocket = [
     "websockets>=12.0",
 ]
+sixel = [
+    "pysixel>=0.1.0",
+]
 browser = [
     "playwright>=1.40.0",
 ]