M4: CLI polish — version flag, mountpoint override, stable aliases

Add --version flag with build-time injection via ldflags. Add
--mountpoint/-m flag to mount for one-off mount point override. Change
init aliases from device path basename (sda1) to UUID prefix (b8b2f8e3)
for stability across boots. Add .gitignore. Update flake.nix with
version injection.

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

cmd/init.go

@@ -6,6 +6,7 @@ import (
 	"path/filepath"
 	"strings"
 
+
 	"git.wntrmute.dev/kyle/arca/internal/config"
 	"git.wntrmute.dev/kyle/arca/internal/udisks"
 	"github.com/godbus/dbus/v5"
@@ -62,7 +63,7 @@ func runInit(cmd *cobra.Command, args []string) error {
 			continue
 		}
 
-		alias := aliasFromPath(dev.DevicePath)
+		alias := aliasFromUUID(dev.UUID)
 		cfg.Devices[alias] = config.DeviceConfig{
 			UUID:    dev.UUID,
 			Methods: []string{"passphrase"},
@@ -101,8 +102,12 @@ func isRootBacking(path dbus.ObjectPath, rootDevices []dbus.ObjectPath) bool {
 	return false
 }
 
-func aliasFromPath(devPath string) string {
-	// "/dev/sda1" -> "sda1", "/dev/nvme0n1p2" -> "nvme0n1p2"
-	base := filepath.Base(devPath)
-	return strings.TrimPrefix(base, "dm-")
+func aliasFromUUID(uuid string) string {
+	// Use first 8 chars of UUID as a stable alias.
+	// "b8b2f8e3-4cde-4aca-a96e-df9274019f9f" -> "b8b2f8e3"
+	clean := strings.ReplaceAll(uuid, "-", "")
+	if len(clean) > 8 {
+		clean = clean[:8]
+	}
+	return clean
 }