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/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