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.

internal/assets/assets.go

@@ -1,25 +1,72 @@
-// Package assets hält die eingebetteten Presets und Custom-Scripte.
+// Package assets hält die eingebetteten Presets und Custom-Scripte sowie
+// den Drop-in-Ordner für eigene Vorlagen (~/.config/motd-assist/templates).
 package assets
 
 import (
 	"embed"
+	"os"
+	"path/filepath"
+	"sort"
 	"strings"
 )
 
 //go:embed presets scripts
 var embedded embed.FS
 
-// ScriptContent liefert den Inhalt einer gebündelten Script-Vorlage.
-func ScriptContent(name string) (string, bool) {
-	b, err := embedded.ReadFile("scripts/" + name)
-	if err != nil {
-		return "", false
-	}
-	return string(b), true
+// Template ist eine installierbare Script-Vorlage.
+type Template struct {
+	Name   string
+	Source string // "builtin" oder "user"
 }
 
-// ScriptNames listet alle gebündelten Script-Vorlagen.
-func ScriptNames() []string {
+// TemplatesDir liefert den Drop-in-Ordner für eigene Vorlagen (XDG-konform).
+func TemplatesDir() (string, error) {
+	if d := os.Getenv("XDG_CONFIG_HOME"); d != "" {
+		return filepath.Join(d, "motd-assist", "templates"), nil
+	}
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(home, ".config", "motd-assist", "templates"), nil
+}
+
+// EnsureTemplatesDir legt den Drop-in-Ordner an, falls er fehlt.
+func EnsureTemplatesDir() (string, error) {
+	dir, err := TemplatesDir()
+	if err != nil {
+		return "", err
+	}
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return "", err
+	}
+	return dir, nil
+}
+
+// userTemplates liest den Drop-in-Ordner frisch bei jedem Aufruf, damit
+// neue Dateien ohne Neustart erkannt werden (TUI: Taste r).
+func userTemplates() map[string]string {
+	dir, err := TemplatesDir()
+	if err != nil {
+		return nil
+	}
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		return nil
+	}
+	out := make(map[string]string, len(entries))
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		if b, err := os.ReadFile(filepath.Join(dir, e.Name())); err == nil {
+			out[e.Name()] = string(b)
+		}
+	}
+	return out
+}
+
+func embeddedScriptNames() []string {
 	entries, err := embedded.ReadDir("scripts")
 	if err != nil {
 		return nil
@@ -33,6 +80,78 @@ func ScriptNames() []string {
 	return names
 }
 
+func embeddedContent(name string) (string, bool) {
+	b, err := embedded.ReadFile("scripts/" + name)
+	if err != nil {
+		return "", false
+	}
+	return string(b), true
+}
+
+// Templates listet alle bekannten Vorlagen sortiert. Gleichnamige
+// User-Vorlagen überdecken Built-ins.
+func Templates() []Template {
+	user := userTemplates()
+	seen := make(map[string]string, len(user)+4)
+	for name := range user {
+		seen[name] = "user"
+	}
+	for _, name := range embeddedScriptNames() {
+		if _, ok := seen[name]; !ok {
+			seen[name] = "builtin"
+		}
+	}
+	out := make([]Template, 0, len(seen))
+	for name, source := range seen {
+		out = append(out, Template{Name: name, Source: source})
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].Name < out[j].Name })
+	return out
+}
+
+// ScriptNames listet alle Vorlagen-Namen (Union aus Built-in und Drop-in).
+func ScriptNames() []string {
+	names := make([]string, 0)
+	for _, t := range Templates() {
+		names = append(names, t.Name)
+	}
+	return names
+}
+
+// ScriptContent liefert den Inhalt einer Vorlage; User-Vorlagen aus dem
+// Drop-in-Ordner haben Vorrang vor Built-ins.
+func ScriptContent(name string) (string, bool) {
+	if c, ok := userTemplates()[name]; ok {
+		return c, true
+	}
+	return embeddedContent(name)
+}
+
+// MatchScript findet die Vorlage, deren Inhalt zu content passt, und
+// erkennt damit bereits installierte motd-assist-Scripte.
+func MatchScript(content string) (string, bool) {
+	return MatchScriptBytes([]byte(content))
+}
+
+// MatchScriptBytes ist die Byte-Variante von MatchScript.
+func MatchScriptBytes(b []byte) (string, bool) {
+	content := strings.TrimSpace(string(b))
+	if content == "" {
+		return "", false
+	}
+	for name, c := range userTemplates() {
+		if strings.TrimSpace(c) == content {
+			return name, true
+		}
+	}
+	for _, name := range embeddedScriptNames() {
+		if c, ok := embeddedContent(name); ok && strings.TrimSpace(c) == content {
+			return name, true
+		}
+	}
+	return "", false
+}
+
 // PresetFiles liefert Dateiname → Inhalt aller eingebetteten Presets.
 func PresetFiles() map[string]string {
 	entries, err := embedded.ReadDir("presets")
@@ -50,23 +169,3 @@ func PresetFiles() map[string]string {
 	}
 	return files
 }
-
-// MatchScript findet die Vorlage, deren Inhalt zu content passt, und
-// erkennt damit bereits installierte motd-assist-Scripte.
-func MatchScript(content string) (string, bool) {
-	return MatchScriptBytes([]byte(content))
-}
-
-// MatchScriptBytes ist die Byte-Variante von MatchScript.
-func MatchScriptBytes(b []byte) (string, bool) {
-	content := strings.TrimSpace(string(b))
-	if content == "" {
-		return "", false
-	}
-	for _, name := range ScriptNames() {
-		if c, ok := ScriptContent(name); ok && strings.TrimSpace(c) == content {
-			return name, true
-		}
-	}
-	return "", false
-}