fix(cli): apply focus option for external tmux sessions

[piersolenski]

Description

Fixes #179

When using tmux with create = "split" or create = "window", the focus option was not being applied because external tmux sessions don't have a Neovim terminal window. The existing focus logic only worked for terminal-based sessions.

Manual Testing

With the following configuration:

cli = {
  mux = {
    backend = "tmux",
    enabled = true,
    create = "split",
  },
}

Running require("sidekick.cli").toggle({ name = "opencode", focus = true }) or require("sidekick.cli").focus() now correctly focuses the tmux pane.