#3186 Docs: worker connect no longer needs a swamp-club credential (swamp-club#3128)

swamp-club#3128 lets a worker pass the auth gate on the pass of the serve that enrolls it, so worker pods need no swamp-club key.

Manual pages to update:

• content/manual/reference/swamp-account-requirement.md: line 35 says every other command needs a credential including swamp worker; worker connect is now the exception (it may pass through its serve). Line 168 says containers set SWAMP_API_KEY and SWAMP_SIGNIN_TOKEN; for worker containers that is now optional. Add a short Remote workers paragraph: the serve sends its pass at enrollment; the worker checks swamp-club signed it and it had not expired; an older serve sends none, so upgrade the serve or give the worker a key; a keyless worker cannot run commands that call swamp-club itself (extension push, issue).
• content/manual/how-to/worker-fleets (docker, docker-compose, kubernetes): drop any instruction to mount a swamp-club key into worker pods, or mark it optional.

Source of truth: design/surfaces/auth-gate.md, section Remote workers, in the swamp repo.