When the gateway is installed as a macOS launch agent and no token is
configured, the service enters an infinite restart loop because launchd
does not inherit shell environment variables. Auto-generate a token
during `gateway install` when auth mode is `token` and no token exists,
matching the existing pattern in doctor.ts and configure.gateway.ts.
The token is persisted to the config file and embedded in the plist
EnvironmentVariables for belt-and-suspenders reliability.
Relates-to: #5103, #2433, #1690, #7749
94d6858160