docs: document SSSD/Authentik directory auth and secrets

README.md

@@ -52,6 +52,33 @@ The home config is also exposed for use beyond these hosts:
   (`inputs.<this>.homeModules.default`). Consumers must supply the module args
   these expect: `inputs` always, `identity` for git/desktop, `portable` for sway.
 
+## Directory authentication (SSSD → Authentik LDAP)
+
+Every NixOS host authenticates users against the Authentik LDAP outpost via
+SSSD, implemented in [`modules/sssd.nix`](./modules/sssd.nix) and enabled by
+default through the `services.authentikLdap.enable` option (added to
+`baseModules`). The **EDaaS** WSL box opts out
+(`services.authentikLdap.enable = false`) as a work-managed environment; the
+macOS host is unaffected (SSSD is Linux-only).
+
+- Connects over LDAPS to `ldap.lyrapup.pet:636`, search base
+  `dc=ldap,dc=goauthentik,dc=io`, binding as
+  `cn=sssd-bind,ou=users,dc=ldap,dc=goauthentik,dc=io`.
+- The schema mappings match Authentik's non-standard object classes
+  (`goauthentik.io/ldap/user`, `goauthentik.io/ldap/group`) over the POSIX
+  attributes (`uid`, `uidNumber`, `gidNumber`, `homeDirectory`).
+- Home directories are created on first login (`pam_mkhomedir`).
+
+### Secrets (agenix)
+
+The LDAP bind credential is an [agenix](https://github.com/ryantm/agenix)
+secret, decrypted at activation with each host's SSH host key. The decrypted
+plaintext is a full `sssd.conf` drop-in delivered to
+`/etc/sssd/conf.d/01-ldap-authtok.conf`, so the password never enters the Nix
+store. Owner setup (host recipient keys, encrypting the bind password, DNS for
+`ldap.lyrapup.pet`, rebuild) is documented in
+[`secrets/README.md`](./secrets/README.md).
+
 ## Applying
 
 ```sh