channels: add missing scry and update usage in activity

desk/app/activity.hoon

@@ -1180,8 +1180,7 @@
   =.  importing  &
   =.  indices   (~(put by indices) [%base ~] *index:a)
   =.  cor  set-chat-reads
-  ::REVIEW  maybe need a scry api version bump here?
-  =+  .^(=channels:c %gx (scry-path %channels /v2/channels/full/noun))
+  =+  .^(=channels:c %gx (scry-path %channels /v3/channels/full/noun))
   =.  cor  (set-volumes channels)
   =.  cor  (set-channel-reads channels)
   =.  cor  refresh-all-summaries

desk/app/channels.hoon

@@ -734,6 +734,9 @@
     ::
         [%x %v2 %channels full=?(~ [%full ~])]
       ``channels-2+!>((uv-channels-2:utils v-channels ?=(^ full.pole)))
+    ::
+        [%x %v3 %channels full=?(~ [%full ~])]
+      ``channels-3+!>((uv-channels-3:utils v-channels ?=(^ full.pole)))
     ::
       [%x ?(%v0 %v1) %init ~]    ``noun+!>([unreads (uv-channels-1:utils v-channels)])
       [%x %v2 %init ~]  ``noun+!>([unreads (uv-channels-2:utils v-channels |)])

desk/mar/channels-2.hoon

@@ -9,6 +9,6 @@
   --
 ++  grab
   |%
-  ++  noun  channels:d
+  ++  noun  channels:v1:old:d
   --
 --

desk/mar/channels-3.hoon

@@ -0,0 +1,14 @@
+/-  c=channels
+/+  j=channel-json
+|_  =channels:c
+++  grad  %noun
+++  grow
+  |%
+  ++  noun  channels
+  ++  json  (channels-2:enjs:j channels)
+  --
+++  grab
+  |%
+  ++  noun  channels:c
+  --
+--