calendar: add todo edit command

Allow editing todo fields (due date, priority) in place via todoman,
instead of requiring delete + recreate. Updates all docs for consistency.

skills/calendar/scripts/calendar.sh

@@ -6,6 +6,7 @@
 #   ./calendar.sh reply [options]         # accept/decline/tentative
 #   ./calendar.sh todo add [options]      # create a todo
 #   ./calendar.sh todo list [options]     # list pending todos
+#   ./calendar.sh todo edit [options]     # edit a todo's fields
 #   ./calendar.sh todo complete [options] # mark todo as done
 #   ./calendar.sh todo delete [options]   # remove a todo
 #   ./calendar.sh todo check              # daily digest for cron