Rewrite init.lua to expose setup(), run(), stop(), and toggle() that
coordinate across config, runner, window, and indicator modules. Replace
the placeholder SayHello command in plugin/watchexec.lua with keymaps
(<Leader>wx{t,s,r}), :WatchexecRun/:WatchexecStop/:WatchexecToggle
commands, VimLeavePre cleanup, and VimResized auto-resize.