feat(config): auto-manage user-dots convenience symlink

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.

config.fish

@@ -226,6 +226,22 @@ if status is-interactive
     # Resolve user-dots path. Customize via: set -U __fish_user_dots_path /your/path
     set -q __fish_user_dots_path
         or set -l __fish_user_dots_path "$XDG_CONFIG_HOME/.user-dots/fish"
+    #   ────────────────────── user-dots convenience symlink ───────────────────
+    #   Keep $__fish_config_dir/user-dots pointing at the resolved path so it can
+    #   be browsed from the fish config dir. Git-ignored; a C2 startup side-effect.
+    #   Only ever manages a symlink — never clobbers a real file/dir placed there.
+    if __fish_config_op_enabled __fish_config_op_autoexec
+        set -l __udots_link "$__fish_config_dir/user-dots"
+        if test -d "$__fish_user_dots_path"
+            if test -L "$__udots_link"
+                test (readlink "$__udots_link") != "$__fish_user_dots_path"
+                    and ln -sfn "$__fish_user_dots_path" "$__udots_link"
+            else if not test -e "$__udots_link"
+                ln -s "$__fish_user_dots_path" "$__udots_link"
+            end
+        end
+    end
+
     #   ─────────────────────── Source machine-local config ────────────────────
     #   Sources local.fish if it exists. That file handles sourcing its own
     #   secrets.fish companion when needed.