feat(config-settings): add Dots link toggle for user-dots symlink

Extract the user-dots symlink logic into the __fish_user_dots_link helper and
add a dedicated __fish_user_dots_symlink toggle, surfaced as a 'Dots link' bool
row on the config-settings Paths page. Unlike the opinionated categories it has
no universal/session split — it acts on a literal path.

Setting it falsy (or toggling off) stops symlink generation and removes any
existing link immediately, honoured regardless of the C2 master switch.
Creation remains a C2 startup side-effect. config.fish now calls the helper.

Docs: README, fish-config.md (C2 table, Paths page, machine-local section),
and fish-config.index updated.

functions/__config_settings_draw_value.fish

@@ -57,10 +57,10 @@ function __config_settings_draw_value
     else
         set title "Path Settings"
         set active_idx 3
-        set vars  __fish_scrollback_history_dir __fish_scrollback_history_max_files __fish_user_dots_path
-        set labels "Log dir" "Log max" "Dots path"
-        set types  path int path
-        set hints  "~/.terminal_history" 100 "(default)"
+        set vars  __fish_scrollback_history_dir __fish_scrollback_history_max_files __fish_user_dots_path __fish_user_dots_symlink
+        set labels "Log dir" "Log max" "Dots path" "Dots link"
+        set types  path int path bool
+        set hints  "~/.terminal_history" 100 "(default)" on
     end
     set -l nrows (count $vars)