Replace Xournal++ with Signed for signing PDFs (#14999)

Signed, Omacom's PDF signing app, takes Xournal++'s place in the base
install and the preinstall set, so Remove and Install Preinstalls cover
it too. A migration installs it on existing machines, unless their owner
has removed the preinstalls; Xournal++ stays on machines that already
have it. The Filling out PDFs chapter now walks through Signed, the GUIs
chapter introduces it, the setup tip points to it, and the unused
Xournal++ config is gone.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

test/shell.d/preinstalls-test.sh

@@ -65,7 +65,7 @@ for package in "${restored[@]}"; do
 done
 pass "every preinstall is shipped in omarchy-base.packages"
 
-for package in omacut monologue omacalc omawrite hype; do
+for package in omacut monologue omacalc omawrite hype signed; do
   printf '%s\n' "${restored[@]}" | grep -qxF "$package" ||
     fail "preinstalls cover the Omacom apps" "$package is missing"
 done