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>

docs/ROADMAP.md

@@ -811,8 +811,9 @@ Design/decision records and a running log of shipped work (items marked
   Avahi mDNS), `openrgb` (RGB lighting daemon), and `restic` (scheduled
   daily backup — a small option surface: `repository`/`passwordFile`/`paths`,
   7/4/6 retention). More candidates by area:
-  - ✓ **cloud/sync:** Syncthing (`services.syncthing`); Nextcloud client is a
-    bare tray app (in the app-suite menu, not a service)
+  - ✓ **cloud/sync:** Syncthing (`services.syncthing`); Nextcloud client is
+    an upstream-HM opt-in (`services.nextcloud-client`, commented in the
+    template — installs the app + starts it with the session; #102)
   - ✓ **local AI:** Ollama (`nomarchy.services.ollama`, optional GPU accel)
     ships; LM Studio is a (commented) app-suite package (a bare GUI, no
     service) — pairs with the menu's Ask-Claude philosophy