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.
