feat(remind): add yearly simple date recurrence handling

Add `yearly_simple_date` collector that detects simple yearly
recurrences (`RRULE:FREQ=YEARLY` with no extra constraints) and stores
the month/day pair in a new `simple_yearly` field on `Remind.rem`.

When rendering, events with `simple_yearly` set are emitted as
`REM <Month> <Day> MSG <summary>` instead of the full dated form.

Add `month_of_int` and `string_of_month` helpers in `Utils`, and update
the RRULE dataset comment with UIDs for easier debugging.

bin/utils.ml

@@ -1,6 +1,37 @@
 open Remind_sync
 open Icalendar
 
+type months = Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec
+
+let month_of_int = function
+  | 1 -> Jan
+  | 2 -> Feb
+  | 3 -> Mar
+  | 4 -> Apr
+  | 5 -> May
+  | 6 -> Jun
+  | 7 -> Jul
+  | 8 -> Aug
+  | 9 -> Sep
+  | 10 -> Oct
+  | 11 -> Nov
+  | 12 -> Dec
+  | _ -> failwith "Invalid month number"
+
+let string_of_month = function
+  | Jan -> "Jan"
+  | Feb -> "Feb"
+  | Mar -> "Mar"
+  | Apr -> "Apr"
+  | May -> "May"
+  | Jun -> "Jun"
+  | Jul -> "Jul"
+  | Aug -> "Aug"
+  | Sep -> "Sep"
+  | Oct -> "Oct"
+  | Nov -> "Nov"
+  | Dec -> "Dec"
+
 let spf = Printf.sprintf
 
 let get_uid ev =