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/root.go

@@ -12,6 +12,10 @@ var rootCmd = &cobra.Command{
 	Short: "Mount and unmount LUKS-encrypted volumes",
 }
 
+func SetVersion(v string) {
+	rootCmd.Version = v
+}
+
 func Execute() {
 	if err := rootCmd.Execute(); err != nil {
 		fmt.Fprintln(os.Stderr, err)