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]

meta/hosts/mainframe.conf.yaml

@@ -1,5 +1,14 @@
 # Headless homelab host. No WM, no terminal emulator, no browser.
 # Owns every singleton automation in the fleet.
+# dotbot does NOT carry a defaults block between config files, so each host
+# config has to restate it. Without this, linking a path that already exists
+# as a real file fails instead of replacing it.
+- defaults:
+    link:
+      create: true
+      relink: true
+      force: true
+
 - link:
     ~/.config/git/identity:     hosts/mainframe/git/identity
     ~/.config/paru:             common/paru

meta/hosts/nas.conf.yaml

@@ -1,5 +1,14 @@
 # Headless server. Shell and nvim should feel identical to the workstations;
 # nothing else belongs here. The shortness of this file is the point.
+# dotbot does NOT carry a defaults block between config files, so each host
+# config has to restate it. Without this, linking a path that already exists
+# as a real file fails instead of replacing it.
+- defaults:
+    link:
+      create: true
+      relink: true
+      force: true
+
 - link:
     ~/.config/git/identity: hosts/nas/git/identity
     ~/.config/paru:         common/paru

meta/hosts/sctfw004.conf.yaml

@@ -1,5 +1,14 @@
 # Work laptop. Ubuntu 24.04, Hyprland, user connorjohnstone.
 # No singleton timers here — that is deliberate.
+# dotbot does NOT carry a defaults block between config files, so each host
+# config has to restate it. Without this, linking a path that already exists
+# as a real file fails instead of replacing it.
+- defaults:
+    link:
+      create: true
+      relink: true
+      force: true
+
 - link:
     ~/.config/git/identity:  hosts/sctfw004/git/identity
     ~/.config/kitty:         common/kitty

meta/hosts/system76.conf.yaml

@@ -1,4 +1,13 @@
 # Personal laptop. Hyprland, full desktop.
+# dotbot does NOT carry a defaults block between config files, so each host
+# config has to restate it. Without this, linking a path that already exists
+# as a real file fails instead of replacing it.
+- defaults:
+    link:
+      create: true
+      relink: true
+      force: true
+
 - link:
     ~/.config/git/identity:  hosts/system76/git/identity
     ~/.config/paru:          common/paru