Split config.fish into repo fragments, concatenated by Nix

Break the interactive init into concern-scoped fish files under
modules/fish/config/ (bindings, env, done, path) and assemble them with
lib.concatMapStringsSep + readFile into a single interactiveShellInit, so
home-manager still writes one ~/.config/fish/config.fish. The pieces stay
editable as separate fish files; the rendered file is unchanged.

modules/fish/config.fish

@@ -1,23 +0,0 @@
-# vi-style modal editing on the command line.
-set -g fish_key_bindings fish_vi_key_bindings
-
-set -gx EDITOR nvim
-set -gx VISUAL nvim
-
-# Render man pages through bat.
-set -x MANROFFOPT "-c"
-set -x MANPAGER "sh -c 'col -bx | bat -l man -p'"
-
-# Tune the done plugin: only notify for commands past 10s, at low urgency.
-set -g __done_min_cmd_duration 10000
-set -g __done_notification_urgency_level low
-
-# Prepend ~/.local/bin to PATH when it exists.
-if test -d ~/.local/bin
-    fish_add_path ~/.local/bin
-end
-
-# Apply fish-compatible profile overrides if present.
-if test -f ~/.fish_profile
-    source ~/.fish_profile
-end