flake: install shell completions for mcp

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

flake.nix

@@ -27,6 +27,14 @@
             "-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/mcp completion zsh > $out/share/zsh/site-functions/_mcp
+            $out/bin/mcp completion bash > $out/share/bash-completion/completions/mcp
+            $out/bin/mcp completion fish > $out/share/fish/vendor_completions.d/mcp.fish
+          '';
         };
 
         mcp-agent = pkgs.buildGoModule {