feat(session): first-boot welcome toast (#81)

One-shot user unit fires a dismissible "You're set" notification on the
first graphical session (SUPER+M / SUPER+T / SUPER+? + Network + Doctor),
then writes settings.firstBootShown into the flake checkout. Skips the
live ISO hostname (hosts/live.nix keeps its own toast). Toggle:
nomarchy.firstBootWelcome.enable. checks.first-boot covers the gate.

Verified: V2 — flake check --no-build; checks.first-boot green; local
shim smoke; downstream-template-home ships the unit.

pkgs/nomarchy-first-boot/default.nix

@@ -0,0 +1,11 @@
+# First-session "you're set" toast (nomarchy.firstBootWelcome). A package
+# so checks.first-boot can exercise the gate on a minimal node — same
+# pattern as nomarchy-battery-notify. libnotify + theme-sync stay on PATH
+# (user unit / VM shim), not runtimeInputs.
+{ writeShellApplication, coreutils }:
+
+writeShellApplication {
+  name = "nomarchy-first-boot";
+  runtimeInputs = [ coreutils ];
+  text = builtins.readFile ./nomarchy-first-boot.sh;
+}