feat: show Discord invite link in install completion message

Helps new users discover the community right after installation.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

bin/install.js

@@ -1059,6 +1059,8 @@ function finishInstall(settingsPath, settings, statuslineCommand, shouldInstallS
   const command = isOpencode ? '/gsd-help' : '/gsd:help';
   console.log(`
   ${green}Done!${reset} Launch ${program} and run ${cyan}${command}${reset}.
+
+  ${cyan}Join the community:${reset} https://discord.gg/5JJgD5svVS
 `);
 }