Add rc auto-release timer/service and rc branch worktree to host setup

The rc service builds from the rc branch worktree (/root/omarchy-pkgs-rc,
created by bin/setup) but publishes into the primary checkout's channel tree
via OMARCHY_REPO_ROOT. The timer is a retry backstop: rc builds are normally
triggered immediately over SSH by the release orchestrator.

systemd/omarchy-auto-release-rc.timer

@@ -0,0 +1,12 @@
+[Unit]
+Description=Retry pending rc releases every 6 hours
+
+[Timer]
+# Backstop only: rc builds are normally triggered immediately over SSH by the
+# release orchestrator (touch .sync-needed-rc + systemctl start). This timer
+# retries builds whose state file survived a failure.
+OnCalendar=*-*-* 01,07,13,19:00:00 America/New_York
+Persistent=true
+
+[Install]
+WantedBy=timers.target