fix(treemacs): open last treemacs session if in $HOME or non-project

`treemacs-add-and-display-current-project-exclusively' seems to confuse
some people and please others when it is used in $HOME. To reduce
confusion, SPC o p will now resume from the last session if
default-directory isn't a valid project OR if it's $HOME. It will also
emit a warning informing them of that.

Fix: #7948
Amend: f9dfb7e92a

modules/ui/treemacs/autoload.el

@@ -12,6 +12,8 @@ Use `treemacs' command for old functionality."
   (require 'treemacs)
   (pcase (treemacs-current-visibility)
     (`visible (delete-window (treemacs-get-local-window)))
-    (_ (if (treemacs--find-current-user-project)
+    (_ (let ((project (treemacs--find-current-user-project)))
+         (if (and project (not (file-equal-p project "~")))
              (treemacs-add-and-display-current-project-exclusively)
-         (treemacs)))))
+           (message "No valid project in current buffer; opening last treemacs session")
+           (treemacs))))))