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>

agent/BACKLOG.md

@@ -86,13 +86,6 @@ untracked canonical state file is still detected.
 
 ## NEXT
 
-### 102. Nextcloud Desktop opt-in through Home Manager
-
-Expose and document upstream Home Manager `services.nextcloud-client` in the
-downstream template, including its background-session lifecycle; do not add a
-duplicate Nomarchy wrapper. Pass = the documented opt-in installs the client,
-starts it with the graphical session, and disabling it removes that lifecycle.
-
 ### 103. Live ISO baseline desktop applications
 
 Make Chromium and Firefox, `libreoffice-fresh`, GNOME Text Editor, Amberol, and