Gate the default shell, convert aliases to abbreviations, use vi bindings

Move the login-shell switch behind a new modules.fish.defaultShell
option (default false; neogaia opts in). Convert every non-eza alias to
an abbreviation, set preferAbbrs and pin generateCompletions, and switch
the command line to vi-style editing. Cap navigation at four dots and
drop psmem/psmem10/dir/vdir/please. Comment the installed packages.

hosts/neogaia/default.nix

@@ -37,6 +37,7 @@
 
   # fish as the login shell.
   modules.fish.enable = true;
+  modules.fish.defaultShell = true;
 
   # Locale preferences for the base system.
   time.timeZone = "America/New_York";