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>

main.go

@@ -2,6 +2,9 @@ package main
 
 import "git.wntrmute.dev/kyle/arca/cmd"
 
+var version = "dev"
+
 func main() {
+	cmd.SetVersion(version)
 	cmd.Execute()
 }