mciasctl: convert from flag to cobra

Adds shell completion support (zsh, bash, fish) via cobra's built-in
completion command. All existing behavior and security measures are
preserved.

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

flake.nix

@@ -28,6 +28,14 @@
             "-w"
             "-X main.version=${version}"
           ];
+          postInstall = ''
+            mkdir -p $out/share/zsh/site-functions
+            mkdir -p $out/share/bash-completion/completions
+            mkdir -p $out/share/fish/vendor_completions.d
+            $out/bin/mciasctl completion zsh > $out/share/zsh/site-functions/_mciasctl
+            $out/bin/mciasctl completion bash > $out/share/bash-completion/completions/mciasctl
+            $out/bin/mciasctl completion fish > $out/share/fish/vendor_completions.d/mciasctl.fish
+          '';
         };
       };
     };