feat: drop-in support for custom templates

User templates can now be added without rebuilding: drop a script into
~/.config/motd-assist/templates/ and reference it from a preset via
"install": "<filename>". Same-name files override built-ins; new files
are picked up by the running TUI via 'r'. `motd-assist templates` lists
known templates and creates the drop-in directory.

Apply plans now embed the resolved template content (Plan.ResolveContents)
so the sudo/root process never needs to read user config; snapshots store
installed script content so restore works under sudo as well. Validation
rejects plans with unresolved content.

scripts/e2e-wsl.sh

@@ -45,6 +45,40 @@ echo
 echo "=== list (nach restore) ==="
 MOTD_ASSIST_ROOT=$R /tmp/motd-assist list
 
+echo
+echo "=== Drop-in-Vorlage anlegen (XDG) ==="
+export XDG_CONFIG_HOME=/tmp/xdg-motd
+rm -rf "$XDG_CONFIG_HOME"
+mkdir -p "$XDG_CONFIG_HOME/motd-assist/templates" "$XDG_CONFIG_HOME/motd-assist/presets"
+printf '#!/bin/sh\necho "== Mein eigenes Template =="\nuname -s\n' > "$XDG_CONFIG_HOME/motd-assist/templates/mein-header"
+cat > "$XDG_CONFIG_HOME/motd-assist/presets/custom.json" <<'EOF'
+{
+  "name": "custom",
+  "description": "Drop-in-Preset mit eigener Vorlage",
+  "scripts": [
+    { "name": "00-header", "enabled": false },
+    { "name": "10-mein-header", "enabled": true, "install": "mein-header" }
+  ]
+}
+EOF
+
+echo
+echo "=== templates (Drop-in muss auftauchen) ==="
+MOTD_ASSIST_ROOT=$R /tmp/motd-assist templates
+
+echo
+echo "=== apply custom (User-Vorlage via eingebetteten Plan-Inhalt) ==="
+MOTD_ASSIST_ROOT=$R /tmp/motd-assist apply custom
+
+echo
+echo "=== preview mit Drop-in-Template ==="
+MOTD_ASSIST_ROOT=$R /tmp/motd-assist preview -w 50
+
+echo
+echo "=== restore (Drop-in-Script muss weg) ==="
+MOTD_ASSIST_ROOT=$R /tmp/motd-assist restore
+MOTD_ASSIST_ROOT=$R /tmp/motd-assist list
+
 echo
 echo "=== ls update-motd.d ==="
 ls -la "$R/etc/update-motd.d"