test(auto-theme): #79 V2 — simulate sunset in a VM (was mis-framed V3)

Bernardo rightly pushed back on deferring #79 to a real session: a
simulated sunset IS VM-testable. Add checks.auto-theme, a runNixOSTest
that installs nomarchy-theme-sync, points NOMARCHY_DEFAULT_THEMES at the
presets, stubs the rebuild via NOMARCHY_REBUILD (a marker), configures
the day/night pair + schedule, then WALKS THE VM CLOCK across the
boundary: 19:55 -> 20:05 -> 20:06 -> next-day 07:30, asserting the active
theme goes summer-day -> night (1 rebuild) -> night (idempotent, still 1)
-> day (2 rebuilds).

This isolates the auto decision + apply + rebuild-trigger; the actual
home-manager switch is stubbed (that generic apply path is exercised by
every manual theme change). The only on-hardware residual is the user
timer firing on its OnCalendar schedule, which systemd-analyze validates.

#79 drops its [blocked:hw] tag — it's V2-verified. ROADMAP updated;
lesson recorded in MEMORY (don't default timer/session features to V3:
stub the generic step, simulate the clock).

Verification: V2 PASS (nix build .#checks.x86_64-linux.auto-theme, fresh,
exit 0); V0 flake check green.

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

agent/BACKLOG.md

@@ -28,23 +28,6 @@ in [`docs/ROADMAP.md`](../docs/ROADMAP.md); map in
 
 ## NEXT
 
-### 79. Auto time-of-day theme pair — all agent work shipped `[blocked:hw]`
-Auto light/dark theme switch through the existing engine, one pipeline
-(`VISION § D`). All 3 slices shipped 2026-07-10; design record in ROADMAP.
-Only the real-session check remains (needs a Nomarchy machine, like #76):
-- **CLI** — `nomarchy-theme-sync auto` (idempotent day/night apply;
-  `--which`/`--force`/`--no-switch`); `cmd_apply` core → `apply_named`.
-- **Timer** — `modules/home/autotheme.nix`: user service+timer, install
-  gated on `settings.autoTheme.enable`, `OnStartupSec=1min` +
-  `OnCalendar=*:0/15` + `Persistent`.
-- **Menu** — Look & Feel › Auto theme: toggle, pick day/night, set times
-  (enable rebuilds to install the timer + `auto --force`; disable is
-  instant `--no-switch`; day/night/times live `--no-switch`).
-
-**V3 remaining:** on a real Nomarchy session — enable from the menu, confirm
-the timer switches day↔night across the sunset/sunrise boundary (and the
-live `home-manager switch` from the timer succeeds).
-
 ### 76. Hibernation default — all agent work shipped `[blocked:hw]`
 Hibernation + zram on by default (product intent, Bernardo 2026-07-10).
 **All V0/V1/V2 slices done** (2026-07-10); only the V3 check remains — on a