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

AGENTS.md

@@ -36,12 +36,13 @@ mise run ci             # Full CI pipeline (topics-init + lint + test-cov)
 
 ```bash
 mise run run            # Run mainline (terminal)
-mise run run-poetry     # Run with poetry feed
-mise run run-firehose   # Run in firehose mode
-mise run run-websocket  # Run with WebSocket display only
-mise run run-both       # Run with both terminal and WebSocket
-mise run run-client     # Run both + open browser
-mise run cmd            # Run C&C command interface
+mise run run-poetry    # Run with poetry feed
+mise run run-firehose  # Run in firehose mode
+mise run run-websocket # Run with WebSocket display only
+mise run run-sixel     # Run with Sixel graphics display
+mise run run-both      # Run with both terminal and WebSocket
+mise run run-client    # Run both + open browser
+mise run cmd           # Run C&C command interface
 ```
 
 ## Git Hooks
@@ -128,6 +129,7 @@ The project uses pytest with strict marker enforcement. Test configuration is in
 - **Display abstraction** (`engine/display.py`): swap display backends via the Display protocol
   - `TerminalDisplay` - ANSI terminal output
   - `WebSocketDisplay` - broadcasts to web clients via WebSocket
+  - `SixelDisplay` - renders to Sixel graphics (pure Python, no C dependency)
   - `MultiDisplay` - forwards to multiple displays simultaneously
 
 - **WebSocket display** (`engine/websocket_display.py`): real-time frame broadcasting to web browsers
@@ -135,6 +137,12 @@ The project uses pytest with strict marker enforcement. Test configuration is in
   - HTTP server on port 8766 (serves HTML client)
   - Client at `client/index.html` with ANSI color parsing and fullscreen support
 
+- **Display modes** (`--display` flag):
+  - `terminal` - Default ANSI terminal output
+  - `websocket` - Web browser display (requires websockets package)
+  - `sixel` - Sixel graphics in supported terminals (iTerm2, mintty, etc.)
+  - `both` - Terminal + WebSocket simultaneously
+
 ### Command & Control
 
 - C&C uses separate ntfy topics for commands and responses