feat: run shell commands asynchronously from command mode

src/main.rs

@@ -61,6 +61,8 @@ Command: Tab/Shift+Tab completes command names and cycles forward/backward with
 Search: Enter accepts current/next literal match, Esc cancels (case-sensitive, wraps)
 Commands: :save [PATH], :force_save [PATH], :quit, :force_quit,
           :save_and_quit [PATH], :force_save_and_quit [PATH], :plugins, :PLUGIN [ARGS]
+Shell: :!COMMAND runs asynchronously in the project directory using $SHELL (or /bin/sh)
+       Pending command/spinner in top bar; output/exit status in floating window; Enter/Esc closes
 Aliases: :w=save, :w!=force_save, :q=quit, :q!=force_quit,
          :wq=save_and_quit, :wq!=force_save_and_quit
 Save actions: core.save, core.force_save, core.save_and_quit, core.force_save_and_quit