fix: spec review - add missing fish module to base/shell

- Create home/base/shell/fish.nix
- Add to base/shell/default.nix imports
- Migrate remaining hosts from features.cli.fish to base.shell.fish

home/base/shell/default.nix

@@ -1,7 +1,8 @@
-# Shell aggregator — imports Nushell (primary) and Starship prompt.
+# Shell aggregator — imports Nushell (primary), Fish, and Starship prompt.
 {...}: {
   imports = [
     ./nushell.nix
+    ./fish.nix
     ./starship.nix
   ];
 }