org-agenda: New customization `org-agenda-start-with-archives-mode'

* lisp/org-agenda.el: Add `org-agenda-start-with-archives-mode'.
(org-agenda-mode): Set value of `org-agenda-archive-mode' according to
value of new variable `org-agenda-start-with-archives-mode'.

* doc/org-manual.org: Note new variable in the documentation of
`org-agenda-archives-mode'.

* etc/ORG-NEWS: Note change to add
`org-agenda-start-with-archives-mode'.

doc/org-manual.org

@@ -10077,9 +10077,13 @@ the other commands, point needs to be in the desired line.
 
   #+kindex: v a
   #+findex: org-agenda-archives-mode
+  #+vindex: org-agenda-start-with-archives-mode
   Toggle Archives mode.  In Archives mode, trees that are archived
   (see [[*Internal archiving]]) are also scanned when producing the
-  agenda.  To exit archives mode, press {{{kbd(v a)}}} again.
+  agenda.  To exit archives mode, press {{{kbd(v a)}}} again.  The
+  initial setting for this mode in new agenda buffers can set with the
+  variable ~org-agenda-start-with-archives-mode~, which can be set
+  with the same values as ~org-agenda-archives-mode~.
 
 - {{{kbd(v A)}}} ::
 

etc/ORG-NEWS

@@ -693,6 +693,11 @@ manner with ~run-python~.
 
 This allows to run functions after ~org-indent~ intializes a buffer to
 enrich its properties.
+*** New option ~org-agenda-start-with-archives-mode~
+
+This option starts the agenda to automatically include archives,
+propagating the value for this variable to ~org-agenda-archives-mode~.
+For acceptable values and their meaning, see the value of that variable.
 
 ** New features
 *** =ob-plantuml.el=: Support tikz file format output

lisp/org-agenda.el

@@ -1102,6 +1102,14 @@ removed from entry text before it is shown in the agenda."
   :group 'org-agenda
   :type 'string)
 
+(defcustom org-agenda-start-with-archives-mode nil
+  "Initial value of archive-mode in a newly created agenda window.
+See `org-agenda-archives-mode' for acceptable values and their
+meaning."
+  :group 'org-agenda-startup
+  :package-version '(Org . "9.7")
+  :type 'symbol)
+
 (defvar org-agenda-entry-text-cleanup-hook nil
   "Hook that is run after basic cleanup of entry text to be shown in agenda.
 This cleanup is done in a temporary buffer, so the function may inspect and
@@ -2372,7 +2380,8 @@ The following commands are available:
     (setq org-agenda-follow-mode org-agenda-start-with-follow-mode
 	  org-agenda-entry-text-mode org-agenda-start-with-entry-text-mode
 	  org-agenda-show-log org-agenda-start-with-log-mode
-	  org-agenda-clockreport-mode org-agenda-start-with-clockreport-mode))
+	  org-agenda-clockreport-mode org-agenda-start-with-clockreport-mode
+          org-agenda-archives-mode org-agenda-start-with-archives-mode))
   (add-to-invisibility-spec '(org-filtered))
   (org-fold-core-initialize `(,org-link--description-folding-spec
                               ,org-link--link-folding-spec))