Emit a lock/unlock hook, or make the lock blank configurable

[mystakai]

The lock plugin's display blank is hardcoded in both its timing and its mechanism, and
the plugin reads no shellConfig keys at all (grep -c shellConfig over
shell/plugins/lock/*.qml returns 0 for both files). So there is no supported way to
change the 5-second interval, to change what blanking does, or to exempt an output.

The only supported escape is omarchy plugin clone omarchy.lock, which forks 769 lines
of PAM, session-lock and fingerprint code that then stops receiving upstream security
fixes. That is a steep price for "don't power down one monitor", and it is why our own
workaround for #8863 ended up entirely outside Omarchy — a user systemd unit tailing
the journal for lock events.

Journal-tailing is the wrong interface and it is the only one available: the hook set
is battery-low, font-set, post-boot, post-update, pre-refresh-pacman,
theme-set — no lock or unlock event — and nothing in Omarchy sets logind's
LockedHint, so the standard external signal is unavailable too.

Either of these would remove the need for a fork:

1. A lock / unlock hook, consistent with the existing hook mechanism. This is
   the more general fix: it serves display quirks, but also per-lock automation people
   currently cannot express at all. (Noctalia has an on-lock hook, which made the same
   workaround straightforward there.)
2. shellConfig keys for the blank — at minimum enable/disable, ideally interval
   and an output exclusion list.

Related bugs, both of which the above would let users work around without forking:
#8847 (the hardcoded 5 s is re-armed on every wake) and #8863 (on link-dropping
monitors that re-arm becomes a self-sustaining flap).