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>

server/src/index.ts

@@ -497,6 +497,18 @@ if (config.heartbeatSchedulerEnabled) {
 
 server.listen(listenPort, config.host, () => {
   logger.info(`Server listening on ${config.host}:${listenPort}`);
+  if (process.env.PAPERCLIP_OPEN_ON_LISTEN === "true") {
+    const openHost = config.host === "0.0.0.0" || config.host === "::" ? "127.0.0.1" : config.host;
+    const url = `http://${openHost}:${listenPort}`;
+    void import("open")
+      .then((mod) => mod.default(url))
+      .then(() => {
+        logger.info(`Opened browser at ${url}`);
+      })
+      .catch((err) => {
+        logger.warn({ err, url }, "Failed to open browser on startup");
+      });
+  }
   printStartupBanner({
     host: config.host,
     deploymentMode: config.deploymentMode,