Add new PERIODIC tag for recurring tasks

As described in [1], we are sometimes representing recurring tasks as lists of
single tasks plus a recurring task to create new instances once in a while.  All
of this is grouped under a common headline, and those headlines should be marked
with PERIODIC to inhibit automatic clock-in.

[1]: https://karl-voit.at/2017/01/15/org-clone-subtree-with-time-shift/

init.el

@@ -778,7 +778,8 @@ With given ARG, display files in `db/important-document-path’."
                   ("FUN" . ?f)
                   ("UNTAGGED" . ?u)
                   (:endgroup . nil)
-                  ("NOTE" . ?n))
+                  ("NOTE" . ?n)
+                  ("PERIODIC" . ?p))
 
                 org-fast-tag-selection-single-key 'expert)
 

site-lisp/db-org.el

@@ -282,8 +282,9 @@ In ~%s~:
            (org-back-to-heading t))
        (while (and (not parent-task)
                    (org-up-heading-safe))
-         (let ((tags (nth 5 (org-heading-components))))
-           (unless (and tags (member "NOP" (split-string tags ":" t)))
+         (let ((tags (org-get-tags nil 'local)))
+           (unless (or (member "NOP" tags)
+                       (member "PERIODIC" tags))
              (setq parent-task (point)))))
        parent-task))))