dotcli: add dot setup folders (XDG short-name migration)

Adds a dot setup subcommand family (idempotent, re-runnable machine-setup
tasks) with a folders task that brings the 8 standard XDG user
directories under a fixed short-name convention (Desktop -> .desktop,
Documents -> doc, Downloads -> dwn, Music -> mus, Pictures -> pic,
Videos -> vid, Templates/Public -> .ignoreme), hardcoded rather than read
from ~/.config/user-dirs.dirs -- that file is a separate, manually
tracked dotfile whose values can drift or go stale, and the migration
must not depend on it being correct. The nested Pictures/Screenshots
folder is renamed to pic/screenshots in the same pass.

Content left behind in a legacy full-named folder by a fresh XDG-defaults
install -- empty or not -- is merged into its short-named replacement.
A same-named collision between a legacy folder and its target is never
overwritten: it is skipped, reported, and the legacy folder is left in
place rather than deleted while something in it could not be merged.
--dry-run previews what would move/skip without touching the filesystem.
xdg-user-dirs-update runs once afterward to notify running apps/portals.

.config/dot/commands/setup/setup.fish

@@ -0,0 +1,35 @@
+function _dot_setup_usage
+    echo "usage: dot setup [<task>]
+
+Tasks:
+  folders   bring the 8 standard XDG user directories under the short-name convention
+  help      show this message
+
+Run 'dot setup <task> help' for details on a specific task.
+
+With no task given, runs every setup task."
+end
+
+function _dot_setup
+    if test "$argv[1]" = help
+        _dot_setup_usage
+        return 0
+    end
+
+    set -l helper_dir (status dirname)
+    source $helper_dir/folders.fish
+
+    if test -z "$argv[1]"
+        _dot_setup_folders
+        return $status
+    end
+
+    switch $argv[1]
+        case folders
+            _dot_setup_folders $argv[2..-1]
+            return $status
+        case '*'
+            _dot_setup_usage
+            return 1
+    end
+end