feat(live-sync): path-activate heartbeat on dotfiles writes (Refs #11)

Timer-only heartbeat left GUI dotfiles edits uncommitted for up to 59
minutes; a trunk-sync session clearing the dirty tree on 2026-10-18
destroyed a month of uncommitted noctalia bar layout inside that gap.
live-sync.path (DirectoryNotEmpty on dotfiles/) fires the existing
flock-guarded oneshot within seconds of any repo-side dotfiles write,
shrinking the exposure window from ~1h to ~2s. Timer stays as the
hourly catch-all for the machine tiers.

dotfiles/systemd-user/.config/systemd/user/live-sync.path

@@ -0,0 +1,13 @@
+[Unit]
+Description=Fire the live-sync heartbeat within seconds of a dotfiles-tier write
+
+[Path]
+# GUI writes (noctalia settings.toml, niri cfg) land in the repo via the
+# dotfiles symlinks; this directory watch makes "GUI edit -> durable commit"
+# sub-second instead of up to one heartbeat interval. DirectoryNotEmpty fires
+# while the dir holds any file, so the unit re-triggers on every modification
+# (the empty-dir edge case is irrelevant here — dotfiles is never empty).
+DirectoryNotEmpty=%h/.omp/dotfiles
+
+[Install]
+WantedBy=paths.target