Set default volume and brightness at login

Add three `systemd` user units that reset
volume, screen brightness, and keyboard backlight,
so that every session starts from a known state.

Values come from ~/.config/environment.d/50-defaults.conf,
and can be overridden per machine via an untracked 90-local.conf

.config/environment.d/50-defaults.conf

@@ -0,0 +1,19 @@
+# Session defaults, read by `systemd --user` at login
+#
+# Applied by:
+#  - set-brightness-default.service
+#  - set-keyboard-brightness-default.service
+#  - set-volume-default.service
+#
+# For machine-local overrides create an untracked
+# ~/.config/environment.d/90-local.conf instead
+# and repeat only the keys you want to change
+#
+# Apply with: `systemctl --user daemon-reload`,
+#              then log out and back in
+#
+# Verify with: `systemctl --user show-environment`
+
+DEFAULT_BRIGHTNESS=30
+DEFAULT_KEYBOARD_BRIGHTNESS=50
+DEFAULT_VOLUME=30

.config/environment.d/README.md

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+# Session Environment Defaults
+
+This folder contains environment variables
+    read by `systemd --user` at login.
+
+Machine-local overrides go into an untracked `90-local.conf`.