feat: adapt spine-add-book for shelf-based nesting

spine.el

@@ -227,13 +227,13 @@ SELECTED-ID expands the matching book's detail section."
                            nil))))))))
           )))))
 (defun spine-add-book (&rest args)
-  "Add a new WANT book to `spine-org-file'.
-Keyword arguments: :title :author :category :format :isbn :cover
+  "Add a new book to shelf `:shelf' in `spine-org-file'.
+Keyword arguments: :title :author :shelf :format :isbn :cover
                    :rec_by :rec_note :date_added :initial_note
-Signals an error on failure."
+:shelf is required. Signals an error if the shelf doesn't exist."
   (let ((title (plist-get args :title))
+        (shelf (plist-get args :shelf))
         (author (plist-get args :author))
-        (category (plist-get args :category))
         (format (plist-get args :format))
         (isbn (plist-get args :isbn))
         (cover (plist-get args :cover))
@@ -243,20 +243,32 @@ Signals an error on failure."
         (initial-note (plist-get args :initial_note)))
     (unless title
       (error "spine-add-book: title is required"))
+    (unless shelf
+      (error "spine-add-book: :shelf is required"))
     (let ((org-file (expand-file-name spine-org-file)))
-      ;; Create file if it doesn't exist
       (unless (file-exists-p org-file)
-        (with-temp-file org-file
-          (insert "#+TODO: WANT(w) READING(r) | READ(d) ABANDONED(a)\n\n")))
+        (error "spine-add-book: file %s does not exist; create shelves first"
+               org-file))
       (with-current-buffer (find-file-noselect org-file)
         (unwind-protect
             (progn
-              (goto-char (point-max))
-              ;; Insert new headline
+              ;; Find the shelf headline
+              (goto-char (point-min))
+              (let ((shelf-found nil))
+                (while (and (not shelf-found)
+                            (re-search-forward
+                             (format "^\\*+[ \t]+%s[ \t]*$" (regexp-quote shelf))
+                             nil t))
+                  (when (= (org-current-level) 1)
+                    (setq shelf-found t)))
+                (unless shelf-found
+                  (error "spine-add-book: shelf \"%s\" not found" shelf)))
+              ;; Go to end of shelf section
+              (org-end-of-subtree)
+              ;; Insert new book headline at level 2
               (org-insert-heading nil t)
+              (org-demote-subtree)
               (insert title)
-              ;; Set TODO state
-              (org-todo "WANT")
               ;; Set properties (only non-empty)
               (when (and author (> (length author) 0))
                 (org-set-property "AUTHOR" author))
@@ -279,9 +291,6 @@ Signals an error on failure."
                         (substring (sha1 (concat title (number-to-string (random)))) 0 4)))
               ;; Set last modified
               (org-set-property "LAST_MODIFIED" (format-time-string "[%Y-%m-%d]"))
-              ;; Set tags
-              (when (and category (> (length category) 0))
-                (org-set-tags (list category)))
               ;; Add initial note
               (when (and initial-note (> (length initial-note) 0))
                 (let ((date (if (and date-added (> (length date-added) 0))