feat(remind): escape special characters in MSG clause body

bin/remind.ml

@@ -121,7 +121,20 @@ let add_through b = function
   | Some d -> Buffer.add_string b (spf " THROUGH %s" (Timedesc.Date.to_rfc3339 d))
   | None -> ()
 
-let add_msg b summary = Buffer.add_string b (spf " MSG %s\n" summary)
+(** Escape special characters in the body of a MSG clause.
+    - '%' must become '%%' (literal percent)
+    - '[' must become '["["]' (a Remind expression that evaluates to the literal string "[") *)
+let escape_msg s =
+  let buf = Buffer.create (String.length s) in
+  String.iter
+    (function
+      | '%' -> Buffer.add_string buf "%%"
+      | '[' -> Buffer.add_string buf {|["["]|}
+      | c -> Buffer.add_char buf c)
+    s;
+  Buffer.contents buf
+
+let add_msg b summary = Buffer.add_string b (spf " MSG %s\n" (escape_msg summary))
 
 let date_of_date_or_datetime (d : Icalendar.date_or_datetime) : Timedesc.Date.t =
   match d with