Drop Ghostty and the herdr TUI alias

Drop Ghostty and the herdr TUI alias Herdr.app is the terminal now. Removes the Ghostty cask, its config and link, and the .zshrc fallback for hosts without the xterm-ghostty terminfo, which only Ghostty set. herdr itself stays: the same binary is the TUI and the daemon Herdr.app attaches to, so only the `h` alias that opened the TUI goes.