Slurm's shared files and users: one path everywhere, accounts as Linux users

slurm.shared mounts the site's NFS export, has an llmgw node export the path
over NFS to the Slurm machines alone (2049 in the firewall), or looks for a
file system the site mounts. slurm.users "llmgw" makes each active account a
Linux user on every Slurm machine: login from its e-mail, a UID from 200000
kept in slurm_users and never reused, its home on the shared path, its SSH
keys from the portal's Profile (or ctl slurm users keys) read by sshd from
/etc/llmgw/ssh-keys. server init --slurm chooses /shared from the first
machine and llmgw's users; --slurm-shared and --slurm-users change that.

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

CHANGELOG.md

@@ -53,6 +53,14 @@ APIs change between minors without a deprecation period.
   machine alone (Slurm takes MySQL or MariaDB only). Slurm's ports
   (6817–6819, srun's 60001–63000) answer the cluster's networks only, as
   llmgw's own do.
+- **Slurm's shared files and users.** `slurm.shared` (`ctl slurm shared`) is
+  one path on every Slurm machine: the site's NFS export mounted, an llmgw
+  node exporting it over NFS (what `server init --slurm` chooses, `/shared`
+  from the first machine), or a file system the site mounts. `slurm.users:
+  llmgw` (`ctl slurm users on`) makes each active account a Linux user on
+  every Slurm machine, the same login and UID (from 200000, never reused)
+  everywhere, its home on the shared path; people add SSH keys in the
+  portal's Profile (`ctl slurm users keys` for an operator).
 
 ### Changed