Point $__fish_config_dir/user-dots at $__fish_user_dots_path on interactive
startup so the private overlay can be browsed from the fish config dir. The
link is created if missing and repointed if the path variable changes, only
ever managing a symlink (never clobbering a real file/dir). Gated as a C2
startup side-effect (__fish_config_op_autoexec) and git-ignored.
Docs: README overlay section and fish-config.md C2 table updated.
rootiest
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