fix xlab-gateway host key in secrets.nix and rekey ...

The active host key on xlab-gateway is the original one
(AAAAII+EKDpU...), not the replacement. Corrected and rekeyed.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

add ylw ed25519 key: agenix access, SSH auth, rekey all secrets ...

- Add ylw's ed25519 public key to secrets.nix admins list
- Re-encrypt all .age secrets so ylw can decrypt
- Add ed25519 key to ye-lw21 authorized SSH keys

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

skydick: enable LDAP-backed NSS for POSIX identity resolution ...

Add users.ldap with nslcd pointed at ldap://10.0.0.1/ for passwd/group
lookups. This is identity-only: loginPam=false keeps SSH/console auth
local, and Samba stays on tdbsam until sambaSamAccount objects exist
in LDAP.

- Add agenix secret for LDAP bind credential (cn=query_user)
- nss_initgroups_ignoreusers ALLLOCAL avoids boot-time NSS deadlock
- Add openldap package for admin ldapsearch/ldapmodify
- Update DATAPOOL.md to reflect LDAP identity model, numeric UID/GID
  in tmpfiles for LDAP-only users, and current auth boundaries

Co-Authored-By: Claude Opus 4.6 <[email protected]>