feat: auto-open browser after onboard completes

Set PAPERCLIP_OPEN_ON_LISTEN env flag when onboard triggers a run, and
open the browser to the UI URL once the server starts listening. Uses
the `open` package for cross-platform browser launching.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cli/src/commands/onboard.ts

@@ -272,6 +272,7 @@ export async function onboard(opts: OnboardOptions): Promise<void> {
   }
 
   if (shouldRunNow && !opts.invokedByRun) {
+    process.env.PAPERCLIP_OPEN_ON_LISTEN = "true";
     const { runCommand } = await import("./run.js");
     await runCommand({ config: configPath, repair: true, yes: true });
     return;