feat(display): add Kitty graphics backend and improve font detection

- Add KittyDisplay using kitty's native graphics protocol
- Improve cross-platform font detection for SixelDisplay
- Add run-kitty mise task for testing kitty backend
- Add kitty_test.py for testing graphics protocol

engine/display/backends/sixel.py

@@ -188,6 +188,88 @@ class SixelDisplay:
         self.cell_width = cell_width
         self.cell_height = cell_height
         self._initialized = False
+        self._font_path = None
+
+    def _get_font_path(self) -> str | None:
+        """Get font path from env or detect common locations (cross-platform)."""
+        import os
+        import sys
+        from pathlib import Path
+
+        if self._font_path:
+            return self._font_path
+
+        env_font = os.environ.get("MAINLINE_SIXEL_FONT")
+        if env_font and os.path.exists(env_font):
+            self._font_path = env_font
+            return env_font
+
+        def search_dir(base_path: str) -> str | None:
+            """Search directory for Geist font."""
+            if not os.path.exists(base_path):
+                return None
+            if os.path.isfile(base_path):
+                return base_path
+            for font_file in Path(base_path).rglob("*"):
+                if font_file.suffix.lower() in (".ttf", ".otf", ".ttc"):
+                    name = font_file.stem.lower()
+                    if "geist" in name and ("nerd" in name or "mono" in name):
+                        return str(font_file)
+            return None
+
+        search_dirs: list[str] = []
+
+        if sys.platform == "darwin":
+            search_dirs.extend(
+                [
+                    os.path.expanduser("~/Library/Fonts/"),
+                    "/System/Library/Fonts/",
+                ]
+            )
+        elif sys.platform == "win32":
+            search_dirs.extend(
+                [
+                    os.path.expanduser(
+                        "~\\AppData\\Local\\Microsoft\\Windows\\Fonts\\"
+                    ),
+                    "C:\\Windows\\Fonts\\",
+                ]
+            )
+        else:
+            search_dirs.extend(
+                [
+                    os.path.expanduser("~/.local/share/fonts/"),
+                    os.path.expanduser("~/.fonts/"),
+                    "/usr/share/fonts/",
+                ]
+            )
+
+        for search_dir_path in search_dirs:
+            found = search_dir(search_dir_path)
+            if found:
+                self._font_path = found
+                return found
+
+        if sys.platform != "win32":
+            try:
+                import subprocess
+
+                for pattern in ["GeistMono", "Geist-Mono", "Geist"]:
+                    result = subprocess.run(
+                        ["fc-match", "-f", "%{file}", pattern],
+                        capture_output=True,
+                        text=True,
+                        timeout=5,
+                    )
+                    if result.returncode == 0 and result.stdout.strip():
+                        font_file = result.stdout.strip()
+                        if os.path.exists(font_file):
+                            self._font_path = font_file
+                            return font_file
+            except Exception:
+                pass
+
+        return None
 
     def init(self, width: int, height: int) -> None:
         self.width = width
@@ -210,12 +292,15 @@ class SixelDisplay:
         img = Image.new("RGBA", (img_width, img_height), (0, 0, 0, 255))
         draw = ImageDraw.Draw(img)
 
-        try:
-            font = ImageFont.truetype(
-                "/usr/share/fonts/truetype/dejavu/DejaVuSansMono.ttf",
-                self.cell_height - 2,
-            )
-        except Exception:
+        font_path = self._get_font_path()
+        font = None
+        if font_path:
+            try:
+                font = ImageFont.truetype(font_path, self.cell_height - 2)
+            except Exception:
+                font = None
+
+        if font is None:
             try:
                 font = ImageFont.load_default()
             except Exception: