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/JOURNAL.md

@@ -19,6 +19,21 @@ Template:
 
 ---
 
+## 2026-07-13 — #102 Nextcloud Desktop opt-in in the template (this commit)
+- **Task:** BACKLOG #102 — document upstream HM `services.nextcloud-client`
+  as a downstream-template opt-in; no Nomarchy wrapper.
+- **Did:** Commented `services.nextcloud-client` block in
+  `templates/downstream/home.nix` (enable + startInBackground), and pointed
+  the bare `# nextcloud-client` package line at it (service = app + session
+  lifecycle; re-comment removes both).
+- **Verified:** V1 — pinned HM 26.05 module confirmed
+  (enable/package/startInBackground); eval test on an uncommented template
+  copy via `lib.mkFlake`: unit ExecStart `nextcloud --background`,
+  WantedBy/PartOf `graphical-session.target`. `nix flake check --no-build`
+  green (includes template-sot + downstream-template-home).
+- **Pending:** nothing — runtime lifecycle is the upstream module's contract.
+- **Next suggestion:** #93/#99/#107 land next (shared live-VM evidence run).
+
 ## 2026-07-13 — User-promoted NEXT queue curation (#102–#112; this commit)
 - **Task:** Audit Bernardo's live-ISO, application, menu, and device follow-ups
   against shipped behavior and curate the executable queue.