issue 797: messenger gains the tell verb for a plain notice

ADR 0380 §8: the operator channel's notify is shaped for conditions (a key that
stays open, escalation, a plain rule that rewrites a subject), so a module has
no way to say a plain notice such as new mail. tell sends a headline and one or
two lines at once to every channel, bypassing routing, the burst window and the
walk hold; it offers Details like any message, is kept in the history, but its
record is marked cleared at once so it never stays open. Its cap (10 a channel
an hour) is its own, apart from notify's and the conditions'.

modules/messenger/module.json

@@ -11,6 +11,7 @@
         "open",
         "history",
         "notify",
+        "tell",
         "asks"
       ],
       "accepts": [
@@ -62,6 +63,7 @@
         "open",
         "history",
         "notify",
+        "tell",
         "asks"
       ]
     }