Guard plugin-add against git transport-helper URLs (backport of #8067)

Backport of the plugin-add URL guard (PR #8067 by @bastidotnet, merged to
quattro as 30471bf3) onto the v4-0-1 release branch.

omarchy-plugin-add cloned a user-supplied git URL without the transport-helper
guard omarchy-theme-install already applies. That guard arrived with #7884,
which is on this branch, but it never touched plugin-add -- so the sibling
command still leaned entirely on git's own protocol.ext.allow=never to keep a
URL like ext::sh -c <cmd> from running a command at clone time.

Stock systems are unaffected: Omarchy sets no protocol.* override, so the git
default holds and there is no live exploit here. This is defense in depth --
it closes the gap #7884 left in the sibling path and drops a silent dependency
on a default the project does not control. Reject ext::/fd:: and leading-dash
forms; https, ssh, scp-style and token-auth URLs still clone, including an
scp-style IPv6 host, which carries :: of its own.

Clean cherry-pick: both files are byte-identical to quattro, so merging v4-0-1
into quattro resolves without a conflict. The guard reads the same as the one
already in bin/omarchy-theme-install on this branch. test/shell passes: 186
files, with the plugin-add suite's new cases all running here -- including the
pty-driven prompt case, which is the only path that reaches the guard's
leading-dash arm.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018DEMYa9UWtroz93DhMTtcV

bin/omarchy-plugin-add

@@ -93,6 +93,15 @@ if [[ -z $url ]]; then
   [[ -n $url ]] || fail "a git URL is required"
 fi
 
+# git reads a leading dash as an option, and `<helper>::<address>` as a remote
+# helper to run at clone time. Reject both so an untrusted URL cannot smuggle a
+# transport helper that executes before the plugin is validated or enabled. An
+# scp-style IPv6 host such as git@[2001:db8::1]:org/repo.git carries `::` too and
+# must still clone.
+if [[ $url == -* || $url =~ ^[A-Za-z0-9][A-Za-z0-9+.-]*:: ]]; then
+  fail "'$url' names a git option or transport helper, not a repository."
+fi
+
 if (( ! ASSUME_YES )); then
   cat >&2 <<WARN