refactor(cli): rename --auto-power-on to --on (-O) on serve

Shorter flag name for daily use. No functional change.

src/cli.rs

@@ -40,7 +40,7 @@ pub enum Commands {
         follow: bool,
 
         /// Power on cluster if switch is off (requires HA config).
-        #[arg(long, default_value = "false")]
+        #[arg(short = 'O', long = "on", default_value = "false")]
         auto_power_on: bool,
 
         /// Sync Docker image across nodes before starting.