refactor(fish): assemble the interactive init in Nix, not at runtime

Fold the done plugin tuning back into config.fish and drop the conf.d
fragment: config.fish is read whole into interactiveShellInit, so Nix
assembles the interactive init at build time instead of fish autoloading
a separate conf.d file. functions/copy.fish stays a function file, the
idiomatic home for a lazily-autoloaded function.

modules/fish/fish.nix

@@ -37,9 +37,6 @@ in
         wget # non-interactive HTTP downloader; backs the wget abbreviation
       ];
 
-      # done's tuning lives in its own conf.d snippet, mirroring ~/.config/fish.
-      xdg.configFile."fish/conf.d/done.fish".source = ./conf.d/done.fish;
-
       programs.fish = {
         enable = true;