feat: initial implementation of iCalendar to Remind converter

- Add project scaffolding (dune, dune-project, opam, .ocamlformat)
- Implement basic parsing and handling of iCalendar events
- Add event predicates for common event types (all-day, timed,
  recurrence, exceptions)
- Add transformation logic to map iCalendar events to Remind format
  (stub implementation)
- Provide utilities for extracting event details and converting
  dates/times
- Set up executable entrypoint and command-line interface using Cmdliner
- Include Remind event type definitions and helpers

bin/remind.ml

@@ -0,0 +1,119 @@
+(* Alias esplicito per chiarezza *)
+type date = Ptime.date
+
+let pp_date fmt (y, m, d) = Format.fprintf fmt "%04d-%02d-%02d" y m d
+
+let show_date (d : date) : string =
+  let y, m, day = d in
+  Printf.sprintf "%04d-%02d-%02d" y m day
+
+type tod = Ptime.Span.t (* secondi da mezzanotte locale *)
+
+let pp_tod = Ptime.Span.pp
+let span_min (m : int) : tod = Ptime.Span.of_int_s (m * 60)
+let span_hm ~(h : int) ~(m : int) : tod = Ptime.Span.of_int_s ((h * 3600) + (m * 60))
+
+let hm_of_tod (t : tod) : int * int =
+  match Ptime.Span.to_int_s t with
+  | None -> invalid_arg "tod out of range"
+  | Some s -> (s / 3600, s mod 3600 / 60)
+
+type weekday =
+  | Mon
+  | Tue
+  | Wed
+  | Thu
+  | Fri
+  | Sat
+  | Sun
+
+type time_spec =
+  | All_day
+  | Timed of {
+      at : tod;
+      duration : Ptime.Span.t option;
+    }
+
+type range = {
+  from_ : date;
+  until : date option;
+}
+(* UNTIL inclusivo *)
+
+type recurrence =
+  | No_recur
+  | Daily of {
+      interval : int;
+      span : range;
+    }
+  | Weekly of {
+      interval : int;
+      days : weekday list;
+      span : range;
+    }
+  | Monthly_dom of {
+      interval : int;
+      day : int;
+      span : range;
+    }
+  | Monthly_nth_weekday of {
+      interval : int;
+      n : int;
+      wday : weekday;
+      span : range;
+    }
+  | Yearly of {
+      month : int;
+      day : int;
+    }
+
+type alarm = { warn_before : Ptime.Span.t }
+
+type meta = {
+  location : string option;
+  url : string option;
+  categories : string list;
+  attendees : string list;
+}
+
+type instance_override = {
+  on : date;
+  cancel : bool;
+  summary : string option;
+  time_spec : time_spec option;
+  notes : string option;
+  alarm : alarm option;
+  meta : meta option;
+}
+
+type series = {
+  summary : string;
+  start : date; (* DTSTART locale *)
+  time_spec : time_spec;
+  recur : recurrence;
+  exdates : date list;
+  overrides : instance_override list;
+  alarm : alarm option;
+  notes : string option;
+  meta : meta;
+}
+
+type info_header = string [@@deriving show]
+type info_value = string [@@deriving show]
+
+type event =
+  | Rem of {
+      date_expr : string;
+      at : tod option;
+      duration : Ptime.Span.t option;
+      warn : Ptime.Span.t option;
+      msg : string;
+      exdates : date list;
+      infos : (info_header * info_value) list;
+    }
+  | Omit of date
+[@@deriving show]
+
+let string_of_date (d : date) : string =
+  let y, m, day = d in
+  Printf.sprintf "%04d-%02d-%02d" y m day