Fix two bugs the mainframe cutover surfaced

dotbot does not carry a `defaults` block between config files, so the
host configs ran without force/relink and refused to replace paths that
already existed as real files — the opencode profiles, the Claude files,
the sunshine config and the old systemd units all failed to link. Each
host config now restates the block.

ddns.timer used OnCalendar=*-*-*/2, which systemd rejects: a day-of-month
step needs an explicit start value. The retired cron line was
`0 2 */2 * *`, so the correct spelling is *-*-1/2.

hosts/mainframe/systemd/ddns.timer

@@ -3,7 +3,9 @@ Description=Dynamic DNS record update, every other day
 ConditionHost=mainframe
 
 [Timer]
-OnCalendar=*-*-*/2 02:00:00
+# Matches the retired cron line: 0 2 */2 * *. systemd needs an explicit
+# start value for the day-of-month step; a bare */2 fails to parse.
+OnCalendar=*-*-1/2 02:00:00
 Persistent=true
 
 [Install]