fix(parallel_installer): hold output lock during cask installs

Cask installs can trigger sudo password prompts that write directly to
/dev/tty. Parallel formula workers could print status messages at the
same moment, causing interleaved output like "Password:Using foo".

Switch @output_mutex from Mutex to Monitor (reentrant) and hold it for
the entire cask install. This blocks other workers' status output while
a cask install is in progress, preventing prompt/status interleaving.
Monitor's reentrancy lets write_output calls inside do_install_entry!
re-acquire the lock on the same thread without deadlocking.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>