refactor(fish): split config.fish into 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/bindings.fish

@@ -0,0 +1,2 @@
+# vi-style modal editing on the command line.
+set -g fish_key_bindings fish_vi_key_bindings

modules/fish/config/done.fish

@@ -0,0 +1,3 @@
+# 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

modules/fish/config/env.fish

@@ -0,0 +1,6 @@
+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'"

modules/fish/config/path.fish

@@ -0,0 +1,9 @@
+# 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