Stop the rc unit failing before the first RC is cut

The rc service fetched and reset /root/omarchy-pkgs-rc in ExecStartPre, but
that worktree does not exist until the first RC creates the rc branch — so on
a freshly set up host the unit failed every five minutes, forever, and showed
up as a failed unit in the timer report.

It now runs bin/auto-release-rc from the main checkout, which always exists:
nothing queued exits silently, no rc branch is a clean no-op, and a missing
worktree is created on demand before handing off to the worktree's own
auto-release.

bin/setup

@@ -229,8 +229,8 @@ else
   elif git -C "$BUILD_ROOT" show-ref --verify --quiet refs/remotes/origin/rc; then
     git -C "$BUILD_ROOT" worktree add --track -b rc "$RC_WORKTREE" origin/rc
   else
-    print_info "No rc branch yet — the first RC cut creates it, and the rc build"
-    echo "    trigger creates this worktree on demand. Nothing to do."
+    print_info "No rc branch yet — the first RC cut creates it, and the rc"
+    echo "    release unit creates this worktree on demand. Nothing to do."
   fi
 fi
 echo ""