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/package.json

@@ -11,19 +11,20 @@
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {
+    "@aws-sdk/client-s3": "^3.888.0",
     "@paperclipai/adapter-claude-local": "workspace:*",
     "@paperclipai/adapter-codex-local": "workspace:*",
     "@paperclipai/adapter-openclaw": "workspace:*",
     "@paperclipai/adapter-utils": "workspace:*",
     "@paperclipai/db": "workspace:*",
     "@paperclipai/shared": "workspace:*",
-    "@aws-sdk/client-s3": "^3.888.0",
     "better-auth": "^1.3.8",
     "detect-port": "^2.1.0",
     "dotenv": "^17.0.1",
     "drizzle-orm": "^0.38.4",
     "express": "^5.1.0",
     "multer": "^2.0.2",
+    "open": "^11.0.0",
     "pino": "^9.6.0",
     "pino-http": "^10.4.0",
     "pino-pretty": "^13.1.3",