feat(autotheme): #79 slice 2 — systemd timer runs auto

modules/home/autotheme.nix installs a user service + timer
(nomarchy-auto-theme) that periodically runs `nomarchy-theme-sync auto`,
so the day/night theme is applied on the current schedule through the
same one engine. Install gates on settings.autoTheme.enable (read from
state, the way updates.nix gates on its enable), so a machine not using
the feature carries no timer.

Timer: OnStartupSec=1min (settle after login) + OnCalendar=*:0/15 +
Persistent (catch a transition missed while off). Periodic rather than
exact sunrise/sunset timers — robust to suspend/DST, and `auto` is
idempotent (no rebuild unless the theme must change) so ticks are cheap.

Verification: V1 — real-config eval (state temporarily enabled, then
restored) confirms the units land when enabled (OnCalendar *:0/15,
ExecStart `... nomarchy-theme-sync auto`) and are absent when disabled;
systemd-analyze validates the schedule. V0 flake check green. The live
timer -> home-manager switch on a real Nomarchy session is pending a
Nomarchy machine (same blocker as #76).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

agent/BACKLOG.md

@@ -39,9 +39,15 @@ Auto light/dark theme switch through the **existing** engine (a timer runs
    `--which` prints the decision, `--force`/`--no-switch` too. `cmd_apply`
    core factored into `apply_named` (behavior unchanged). V1 + functional
    tests (day/night/disabled/missing-slugs, apply writes state, idempotent).
-2. **Timer/hook** — systemd user timers at sunrise+sunset running
-   `nomarchy-theme-sync auto`, gated on `settings.autoTheme.enable`; apply
-   the correct theme at session start too. V2 (runNixOSTest).
+2. ~~**Timer/hook**~~ ✓ shipped 2026-07-10 — `modules/home/autotheme.nix`:
+   `systemd.user.{service,timer}.nomarchy-auto-theme`, install gated on
+   `settings.autoTheme.enable` (read from state), `OnStartupSec=1min` +
+   `OnCalendar=*:0/15` + `Persistent`. Chose periodic (not exact
+   sunrise/sunset timers) — robust to suspend/DST, and `auto` is idempotent
+   so ticks are cheap. V1: real-config eval shows units land when enabled
+   (OnCalendar `*:0/15`, ExecStart `… auto`), absent when disabled;
+   systemd-analyze validates the schedule. **Real-session check pending**
+   (live timer→`home-manager switch`) — needs a Nomarchy machine, like #76.
 3. **Menu** — Look & Feel › Auto theme: enable/disable, pick day/night
    themes, set times. Writes state.