docs: rename yt-offline → Catacomb across README, ROADMAP, guides, packaging

Prose + commands updated to the new name: brand reads "Catacomb", while
binary/path/package references are the lowercase `catacomb` (commands, deb/rpm
names, ~/.local/share/catacomb, catacomb.db, catacomb.desktop). Codeberg repo
URLs left pointing at the existing repo. Also folds in the pending doc edits
from earlier in the session.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

yt-offline/hooks/pre-applypatch.sample

@@ -0,0 +1,14 @@
+#!/bin/sh
+#
+# An example hook script to verify what is about to be committed
+# by applypatch from an e-mail message.
+#
+# The hook should exit with non-zero status after issuing an
+# appropriate message if it wants to stop the commit.
+#
+# To enable this hook, rename this file to "pre-applypatch".
+
+. git-sh-setup
+precommit="$(git rev-parse --git-path hooks/pre-commit)"
+test -x "$precommit" && exec "$precommit" ${1+"$@"}
+: