feat(theme-sync): #79 slice 1 — auto day/night theme primitive

First slice of the auto time-of-day theme pair (VISION § D, split into
CLI / timer / menu). `nomarchy-theme-sync auto` reads
settings.autoTheme.{enable,day,night,sunrise,sunset}, picks day vs night
from the local clock, and applies the matching preset — but only if it
differs from the current one, so the slice-2 timer can fire freely
without triggering needless rebuilds. `--which` prints the decision
without switching; `--force` and `--no-switch` included.

cmd_apply's core is factored into apply_named() so `auto` drives the
exact same one engine (no second pipeline). apply behavior is unchanged.

Verification: V1 (package builds) + functional tests on the built binary
against scratch state — wide-day window → day slug, a 00:00–00:01 day
span → night, disabled → no-op, missing day/night → clear die, auto
--no-switch writes state, a re-run no-ops as "already on", and plain
apply still writes (refactor intact). V0 py_compile + flake check +
checks.theme-sync-validate green.

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

agent/BACKLOG.md

@@ -28,6 +28,23 @@ in [`docs/ROADMAP.md`](../docs/ROADMAP.md); map in
 
 ## NEXT
 
+### 79. Auto time-of-day theme pair (`VISION § D`) `[big]` — split
+Auto light/dark theme switch through the **existing** engine (a timer runs
+`nomarchy-theme-sync apply`), one pipeline. State: `settings.autoTheme`
+`{enable, day, night, sunrise, sunset}` in theme-state.json. Slices:
+1. ~~**CLI primitive + schema**~~ ✓ shipped 2026-07-10 —
+   `nomarchy-theme-sync auto` resolves day/night from
+   `settings.autoTheme.{enable,day,night,sunrise,sunset}` and applies (only
+   if different — idempotent, so the timer won't needlessly rebuild);
+   `--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).
+3. **Menu** — Look & Feel › Auto theme: enable/disable, pick day/night
+   themes, set times. Writes state.
+
 ### 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