Pin trusted PATH in privileged DNS helper (backport of #8172)

Backport of the DNS PATH pin (PR #8172 by @mdisec, merged to quattro as
4637735a) onto the v4-0-1 release branch.

omarchy dev link prepends a user-writable checkout's bin/ to sudo's secure_path
so privileged Omarchy commands resolve to the development versions, and that
reaches the subprocesses they launch too. This branch carries the same
passwordless grant -- etc/sudoers.d/omarchy-dns lets wheel run
/usr/bin/omarchy-dns Cloudflare, Google and DHCP without a password -- so the
packaged script ran as root while resolving bare helpers (dirname, install,
tee, rm, nmcli, systemctl, awk) through the caller's secure_path. Write access
to a dev checkout became arbitrary root execution, with no password prompt in
the way.

Pin PATH to trusted system directories once EUID is 0. The restriction lands
only after elevation, so the unprivileged wrapper phase keeps the caller's PATH
and can still find sudo or pkexec; every helper the privileged half uses is a
system utility, so it needs nothing from the checkout.

Clean cherry-pick: both files are byte-identical to quattro, so merging v4-0-1
into quattro resolves without a conflict. Verified by mutation: with the pin
removed, test/shell.d/dns-sudoers-test.sh fails at the poisoned-helper case;
restored, all four of its cases pass. test/shell (189 files) and test/cli pass.

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

bin/omarchy-dns

@@ -6,6 +6,18 @@
 
 set -euo pipefail
 
+# Whenever this runs as root — invoked directly through the passwordless
+# sudoers rule, or re-execed by require_root below — sudo's secure_path decides
+# where a bare helper resolves, and a dev link (etc/sudoers.d/omarchy-dev-path)
+# prepends a user-writable checkout bin/ to it. Every helper this script calls
+# by bare name (dirname, install, tee, rm, nmcli, systemctl, awk) is a system
+# tool, never an omarchy-* command, so pin PATH to trusted system directories
+# and keep root from resolving one out of that checkout. The unprivileged
+# wrapper phase keeps the caller's PATH so it can still find sudo/pkexec.
+if (( EUID == 0 )); then
+  export PATH=/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/sbin:/bin:/sbin
+fi
+
 NM_DNS_CONF=/etc/NetworkManager/conf.d/20-omarchy-dns.conf
 
 provider_from_arg() {