feat: AUTO_UPDATE config option, null next_update_check when disabled

- AUTO_UPDATE=false in config file disables auto-updating
- --no-auto-update flag still works as an override
- next_update_check is omitted from pulses when auto-update is disabled
- status command shows auto-update enabled/disabled state
- config.example documents the new option

config.example

@@ -9,5 +9,8 @@ TOKEN=your-token-here
 # Optional — defaults to https://pulsewise.app/api/collect
 # URL=http://localhost:8000/api/collect
 
+# Optional — set to false to disable automatic updates (default: true)
+# AUTO_UPDATE=false
+
 # Optional — logs full response body from the server
 # DEBUG=true