An agent, so a finger can mean something in your session

The daemon announces every matched finger on the system bus and stops there,
because root has no session bus, no display and no business starting your
applications. fingerprintd-agent is the other half: it runs as you, subscribes
properly rather than parsing gdbus monitor output, filters by uid because the
signal is visible to every local user, and maps fingers to commands from a file
you own and can edit without restarting anything.

It is a separate binary and a separate subpackage because it is a separate
trust domain. /etc/fingerprintd/actions.conf is a root shell and is guarded
like one; ~/.config/fingerprintd/fingers.conf runs your commands as you, so it
is an ordinary dotfile.

Demonstrated on the phone: one press of the unlock finger both unlocks it and
opens plasma-camera.

packaging/fingers.conf.example

@@ -0,0 +1,28 @@
+# fingerprintd-agent — what a finger does in YOUR session.
+#
+# Install as ~/.config/fingerprintd/fingers.conf, then:
+#     systemctl --user enable --now fingerprintd-agent
+#
+# One finger per line:
+#
+#     <finger>  <command...>
+#
+# The command runs as you, with your session's environment, so it can start
+# applications — which is exactly what the daemon cannot do, being root with
+# no session bus and no display.
+#
+# This file is yours. It is re-read whenever it changes, so editing it needs no
+# restart, and unlike /etc/fingerprintd/actions.conf it is not a root shell:
+# relative commands are fine and nothing checks its ownership.
+#
+# Fingers: left-thumb, left-index-finger, left-middle-finger, left-ring-finger,
+# left-little-finger, and the right-* equivalents. A name that is not one of
+# those is reported at startup rather than silently never firing.
+#
+# NOTE: a finger fires this every time it MATCHES, which includes the press
+# that unlocks the phone. If you map your unlock finger, the app opens on
+# every unlock.
+
+#right-ring-finger    kde-open camera
+#left-index-finger    plasma-settings
+#left-thumb           sh -c 'notify-send "hello from a finger"'