feat(state): opt-in auto-commit of state mutations (Phase 4)

settings.autoCommit — toggled from System › Auto-commit (self-gated on
the flake being a git repo) — makes every nomarchy-theme-sync apply/set
also git-commit theme-state.json in the downstream flake, so settings
history is `git log`. Off by default.

Design: the flag is live-read by the tool on each write (nothing in Nix
consumes it → instant toggle, no rebuild, no new option); the commit is
pathspec-limited to theme-state.json so unrelated dirty files are never
swept up; it fires when the flag is on before OR after the write, so
the disable-toggle itself lands in history; same-value writes no-op
(diff against HEAD); missing git identity falls back to
Nomarchy <nomarchy@localhost>; `bg next` is deliberately excluded.

Rider fix: `get` now prints booleans JSON-style (true, not Python's
True) — un-sticking the System menu's "Auto timezone (on/off)" label,
whose `= true` comparison could never match. All shell consumers
already normalise true|True, so no other behavior change.

Verified: V0 (py_compile, nix flake check) + V1 (HM generation builds,
generated nomarchy-menu passes bash -n, wiring present in the rendered
script) + a 7-assertion sandbox-repo round-trip covering enable/set/
same-value/disable/post-disable/apply paths and the identity fallback.
Remaining: on-machine check queued in agent/HARDWARE-QUEUE.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -311,6 +311,14 @@ instant. Internally it's two keys: `settings.nightlight.installed` (sticky —
 gates the unit, the first enable rebuilds) and `settings.nightlight.on` (the
 instant on/off). Expect more `nomarchy.*` toggles to migrate to this pattern.
 
+**Auto-commit (opt-in):** System › Auto-commit makes every `apply`/`set`
+mutation also `git commit` theme-state.json in your flake — *only* that
+file, so unrelated dirty work is never swept up — turning your settings
+history into `git log`. Off by default; the toggle is instant (nothing in
+Nix consumes the flag), the off-write is itself committed so history stays
+consistent, and wallpaper cycling (`bg next`) is deliberately excluded —
+the current wallpaper rides along with the next real commit.
+
 ### Per-theme app assets (`themes/<slug>/`)
 
 Recoloring covers 95% of theming; the rest is one optional assets directory