Send through a bot token so messages can be deleted again

Incoming webhooks return no message id, so nothing can be removed later.
Settings now take a bot token plus channel, chat.postMessage records channel
and timestamp on the run, and the history carries a delete button per
message. The webhook stays as a fallback and is marked as not deletable.

drizzle/meta/_journal.json

@@ -85,6 +85,13 @@
       "when": 1785751667390,
       "tag": "0011_concerned_domino",
       "breakpoints": true
+    },
+    {
+      "idx": 12,
+      "version": "7",
+      "when": 1785753181183,
+      "tag": "0012_solid_tarantula",
+      "breakpoints": true
     }
   ]
 }