fix(tmux): drain kill-pane stdout to prevent pipe backpressure hang

closeTmuxPane spawned kill-pane with stdout: "pipe" but never drained the
stream, which could leave the subprocess hanging indefinitely when tmux
wrote anything to stdout (for example under --force-close race conditions).

- send-keys now uses stdout: "ignore" so there is no pipe to drain
- kill-pane keeps the pipe but drains stdout/stderr alongside proc.exited
- switch imports to the new spawn-process helper so the behavior is
  covered by hermetic tests that mock the spawn boundary

src/shared/tmux/tmux-utils/spawn-process.ts

@@ -0,0 +1 @@
+export { spawn } from "bun"