feat: terminal backend abstraction — kitty, tmux, and plain backends

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/kitty_workbench/backends/kitty.py

@@ -0,0 +1,18 @@
+"""Kitty terminal backend — native splits via kitty @ remote control."""
+from __future__ import annotations
+import subprocess
+from typing import Union
+from kitty_workbench.backends import Backend
+
+class KittyBackend(Backend):
+    name = "kitty"
+    def launch_pane(self, command: list[str], title: str) -> int:
+        result = subprocess.run(
+            ["kitty", "@", "launch", "--location=vsplit", "--title", title] + command,
+            capture_output=True, text=True,
+        )
+        return int(result.stdout.strip())
+    def close_pane(self, pane_id: Union[int, str]) -> None:
+        subprocess.run(["kitty", "@", "close-window", f"--match=id:{pane_id}"], capture_output=True)
+    def image_protocol(self) -> str:
+        return "kitty"