Fix keyboard backlight restore after screensaver dismiss

[marcindyguda]

Problem

After the screensaver shows and is dismissed (before the lock deadline), the keyboard backlight stays dark instead of keeping its previous brightness.

Root cause

• Lock blanking (shell/plugins/lock/Service.qml blankProcess) runs omarchy-brightness-keyboard off = brightnessctl -s set 0 (save + off).
• Wake (bin/omarchy-system-wake) runs omarchy-brightness-keyboard restore = brightnessctl -r.
• But screensaver dismiss (shell/plugins/services/idle/Service.qml cancelIdleCycle) also runs omarchy-system-wake without any preceding off, so restore applied a stale save (often 0 from an earlier cycle) and clobbered the still-lit keyboard. A repeated off while already at zero had the same clobbering effect on the save file.

Reproduced live on a MacBook (kbd_backlight, max 255): stray restore with no preceding off reset a lit keyboard to the stale saved value.

Fix

bin/omarchy-brightness-keyboard now pairs off/restore with a marker file (${XDG_RUNTIME_DIR:-/tmp}/omarchy-keyboard-backlight-off):

• off only saves (-s) when the keyboard is actually lit; a repeated off at zero is a plain set 0 that leaves the save intact.
• restore is a no-op without the marker (screensaver dismiss), one-shot (clears the marker), and leaves an already-lit keyboard alone.

No caller changes needed; omarchy-system-wake keeps calling restore.

Verification

• New test/shell.d/keyboard-backlight-test.sh passes; idle-test.sh passes (bin-style-test.sh failure is pre-existing on clean checkout, unrelated omarchy-remove-ai-openclaw).
• Live check: 25 -> off -> 0 (marker) -> restore -> 25 (marker cleared); stray restore on a lit keyboard is a no-op.