cli: Add hidden command to generate shell autocompletion

default.nix

@@ -16,6 +16,20 @@ in rustPlatform.buildRustPackage {
   };
   cargoSha256 = "1yjaqhv9gd86jq56vsrhv6qv3k5qh2pnc4zyxbi2fm2hdrvy0440";
 
+  postBuild = ''
+    mkdir completions
+    for shell in bash fish zsh; do
+      cargo run --frozen -- gen-completions $shell > completions/$shell
+    done
+  '';
+
+  postInstall = ''
+    mkdir -p "$out/share/"{bash-completion/completions,fish/vendor_completions.d,zsh/site-functions}
+    cp completions/bash $out/share/bash-completion/completions/colmena
+    cp completions/fish $out/share/fish/vendor_completions.d/colmena.fish
+    cp completions/zsh $out/share/zsh/site-functions/_colmena
+  '';
+
   # Recursive Nix is not stable yet
   doCheck = false;
 }