feat: add service field support

- service: true/false is now a required field in manyplug.json
- init --service creates background service template
- list shows ● for services, ○ for standard plugins
- validate checks service field and warns on mismatches
- templates document service vs non-service behavior

Services run regardless of isPluginRunning lock, but can
choose to respect it. Non-services are blocked automatically.

bin/manyplug.js

@@ -22,6 +22,7 @@ program
   .command('init <name>')
   .description('Create a new plugin boilerplate')
   .option('-c, --category <cat>', 'Plugin category', 'utility')
+  .option('-s, --service', 'Create as a background service', false)
   .action(initCommand);
 
 program