Web launcher prints the whole spawn error object when the browser opener cannot start #2645

Problem

Found during the v2.10.0 release smoke (#2623) and recorded in its ledger. When --web cannot spawn the browser opener (no open/xdg-open on PATH), it stays up as intended since #2533, but prints the entire Node error object to its terminal: a stack trace plus spawnargs, which include the launch URL with the API token. That URL is already printed in the banner just above, so nothing new leaks, but the output is noisy and puts the token in a second place for anyone copying the error.

Reproduce

env -i PATH=/var/empty MCP_AUTO_OPEN_ENABLED=true "$(command -v node)" clients/launcher/build/index.js --web
# → Could not open a browser automatically: Error: spawn open ENOENT … (stack, spawnargs with ?MCP_INSPECTOR_API_TOKEN=…)

Expected

One line, as the TUI already prints: Could not open a browser automatically (spawn open ENOENT). Open the URL above manually. No stack, no spawnargs.

Priority

Low (rubric total 5): Severity 2, Urgency 1, +1 bug, +1 milestoned.