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>

flake.nix

@@ -10,13 +10,17 @@
     let
       system = "x86_64-linux";
       pkgs = nixpkgs.legacyPackages.${system};
+      version = "1.0.0";
     in
     {
       packages.${system}.default = pkgs.buildGoModule {
         pname = "arca";
-        version = "0.1.0";
+        inherit version;
         src = ./.;
         vendorHash = null;
+        ldflags = [
+          "-X main.version=${version}"
+        ];
       };
     };
 }