feat(template): document Nextcloud client opt-in via upstream HM service

BACKLOG #102: a commented services.nextcloud-client example in the
downstream template (enable + startInBackground) instead of only the
bare package line — the upstream Home Manager module installs the app
AND ties it to graphical-session.target, so one block covers install,
session autostart, and clean removal. No Nomarchy wrapper added.

Verified: V1 — pinned HM 26.05 module options confirmed; eval test of
the uncommented block through lib.mkFlake shows the user unit
(ExecStart nextcloud --background, WantedBy/PartOf
graphical-session.target); nix flake check --no-build green
(template-sot, downstream-template-home included). Runtime lifecycle is
the upstream module's contract.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

templates/downstream/home.nix

@@ -76,6 +76,12 @@
   # nomarchy.keyboard.layouts = [ "de" "fr" ]; # optional: put these first in the
   #                                            # new-keyboard picker (all installed XKB
   #                                            # layouts are searchable without this)
+  #
+  # services.nextcloud-client = {      # Nextcloud sync client (upstream Home Manager
+  #   enable = true;                   #   module — installs the client and starts it
+  #   startInBackground = true;        #   with the graphical session; tray icon only,
+  # };                                 #   no window). Re-comment to remove the app
+  #                                    #   AND its background service.
 
   # ── Application suite ───────────────────────────────────────────────
   # A starter complete-workstation set, installed for your user — yours to
@@ -170,7 +176,8 @@
     # mangohud                    # in-game overlay
 
     # Files, sync & torrents
-    # nextcloud-client
+    # nextcloud-client            # bare app — services.nextcloud-client above
+    #                             # also autostarts it with the session
     # localsend                   # AirDrop-like transfer
     # qbittorrent