M8: add command to append a single device to config

New 'arca add <device>' subcommand detects a LUKS device via udisks2 and
appends it to the config with passphrase as default method. Supports
--alias/-a to override the generated name. Skips if UUID already
configured. Adds Config.Save() and Config.HasUUID() to config package.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/config/config.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 
@@ -76,6 +77,31 @@ func resolvedFrom(dev DeviceConfig) ResolvedDevice {
 	}
 }
 
+// HasUUID returns true if a device with the given UUID is already configured.
+func (c *Config) HasUUID(uuid string) bool {
+	for _, dev := range c.Devices {
+		if dev.UUID == uuid {
+			return true
+		}
+	}
+	return false
+}
+
+// Save writes the config to the config file.
+func (c *Config) Save() error {
+	path := configPath()
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return fmt.Errorf("creating config directory: %w", err)
+	}
+
+	data, err := yaml.Marshal(c)
+	if err != nil {
+		return fmt.Errorf("marshaling config: %w", err)
+	}
+
+	return os.WriteFile(path, data, 0o644)
+}
+
 // AliasFor returns the config alias for a given UUID, or "" if none.
 func (c *Config) AliasFor(uuid string) string {
 	for name, dev := range c.Devices {