Slurm users reach a node only through a job of theirs

llmgw's Slurm users log in to the controller freely and to another Slurm
machine only while a job of theirs runs there: the agent installs
pam_slurm_adopt and puts it in sshd's account stack for the users it made
alone, so root and the machine's own users are never locked out. Their
session is put in the job's extern step (PrologFlags=Contain), and
pam_systemd, which would move it out, is skipped for them by taking the
session file into sshd's stack with a guard in front of it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/reference.md

@@ -3595,8 +3595,9 @@ Takes a [SharedStorage](#sharedstorage) block.
 Users is who keeps the Linux users that run jobs, the same on every
 machine of the Slurm cluster: "llmgw" makes one for each active llmgw
 account (its login from the e-mail address, a UID of llmgw's from
-200000 up, its SSH keys from the portal); empty leaves users to the
-site (a directory through sssd, or by hand).
+200000 up, its SSH keys from the portal), let in to the controller,
+and to a node only while a job of theirs runs there (pam_slurm_adopt);
+empty leaves users to the site (a directory through sssd, or by hand).
 
 ## StrataConfig