fix: [sc-83534] Auto updater fails to restart service

[mlataza]

Summary

Story details: https://app.shortcut.com/rewst/story/83534

• Fixed a deadlock in the auto-updater where the service process blocks indefinitely on exec.Command(...).Run() waiting for the updater child process to exit, while the updater child is waiting for the service to stop — creating a circular dependency
• Changed .Run() to .Start() in cmd/agent_smith/service.go so the updater process is launched in the background, allowing the service to respond to the stop signal when the updater issues it

Root Cause

The auto-updater flow:

1. Service process (parent) calls exec.Command(path, args...).Run() which blocks until the child exits
2. Updater process (child) runs --update, which calls svc.Stop() to stop the parent service
3. The parent can't stop because it's blocked waiting for the child — deadlock

Fix

Using .Start() instead of .Run() fires the updater process without waiting for it to complete. The updater independently handles stopping the service, copying the new binary, and restarting the service.