ci(build): drop dudumini (x86_64-darwin) from the build matrix

[emaiax]

Summary

• nixpkgs-unstable dropped x86_64-darwin support in the 26.11 cycle (see PR chore(deps): update dependencies #87), breaking every dudumini CI build.
• Per-host nixpkgs pinning was explored in PR fix(nix): pin dudumini to nixpkgs-26.05-darwin, keep dudupro on unstable #89 (closed): nix-darwin's own internal branch-match checks (documentation module, darwin-uninstaller package) each make their own nested eval-config.nix call that ignores per-host overrides, comparing against that host's actual pkgs.lib. Making dudumini track an older stable nixpkgs while nix-darwin/home-manager/sops-nix stay on unstable is unworkable without a second, matching nix-darwin input.
• Decommissioning dudumini from CI for now. nix/hosts/dudumini.nix and its inventory entry are left in place; only the build matrix job is removed.

Test plan

• nix flake check unaffected (lint job only touches formatting/flake metadata, not the build matrix).
• Reviewed workflow diff: only removes the dudumini/macos-15-intel matrix entry, dudupro job untouched.