Share the git URL check, and refuse the transports Omarchy does not clone from (#8174)

* Share the git URL check between theme-install and plugin-add

Both commands clone a URL a stranger can choose, and each carried its own copy of the rule that refuses a git option or a `<helper>::<address>` transport helper before cloning. Two copies of a security check drift: the second one arrived four months after the first, and only because someone went looking for it.

The rule now lives in omarchy-git-url-check and the callers ask it. Its absence refuses the URL rather than waving it through, since the callers read a non-zero status as a refusal and a missing command exits 127.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Co-Authored-By: Codex XHigh <noreply@openai.com>

* Refuse a git URL naming a transport Omarchy does not clone from

`<helper>::<address>` is only one of the two ways a URL reaches a remote helper. git also resolves git-remote-<scheme> for `<scheme>://<address>` whenever the scheme is not one it connects itself, so `ext::sh -c id` and `ext://sh -c id` arrive at the same helper while only the first was refused.

That shape cannot be refused outright, because it is also how every legitimate URL arrives, so the scheme is checked against the transports git still connects itself. `git+ssh` and `ssh+git` are on that list: they are spelled like a helper and read as plain ssh, and leaving them off would refuse a URL that clones today. `ext` and `fd` are off it deliberately -- git ships a helper for each, and `ext` runs whatever command the URL carries.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Co-Authored-By: Codex XHigh <noreply@openai.com>

---------

Co-authored-by: David Heinemeier Hansson <david@hey.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
Co-authored-by: Codex XHigh <noreply@openai.com>

test/shell.d/plugin-add-test.sh

@@ -111,6 +111,20 @@ for bad in "ext::sh -c touch /tmp/omarchy-guard-test" "fd::17"; do
 done
 pass "plugin add rejects transport-helper URLs before cloning"
 
+# The `://` spelling of the same thing: git resolves git-remote-<scheme> for any
+# scheme it does not implement itself, so `ext::` and `ext://` reach the same
+# helper and both have to be refused.
+for bad in "ext://sh -c id" "gcrypt://example.com/x"; do
+  rm -f "$clone_marker"
+  output=$(add_url "$bad") &&
+    fail "plugin add rejects a transport-scheme URL: $bad" "$output"
+  grep -qF "which Omarchy does not clone from" <<<"$output" ||
+    fail "plugin add names the transport-scheme rejection: $bad" "$output"
+  [[ ! -e $clone_marker ]] ||
+    fail "plugin add reached git clone for a transport-scheme URL: $bad"
+done
+pass "plugin add rejects transport-scheme URLs before cloning"
+
 # Option-shaped URLs on argv are refused before clone — by the option parser
 # (`-*` falls to "unknown add option"), not the guard. The guard's own
 # leading-dash arm is only reachable through the interactive gum prompt and is