Read the non-trivial fish snippets from files

Move the interactiveShellInit block and the copy function body into
modules/fish/*.fish and pull them in with builtins.readFile, so the
non-trivial fish lives in real fish files (highlighting, fish_indent)
instead of inline Nix strings. The Auto-loader only collects .nix, so
the new .fish files are ignored by it. Rendered config is unchanged.

modules/fish/interactive-init.fish

@@ -0,0 +1,23 @@
+# 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