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]>