feat: support multiple input files

- Accept multiple positional arguments instead of a single required file
- Extract `read_file` helper to separate file reading from parsing
- Collect all valid reminders across files before printing
- Handle per-file errors gracefully without aborting the whole run

bin/commandLine.ml

@@ -1,17 +1,16 @@
 open Cmdliner
 open Cmdliner.Term.Syntax
 
-let ical_file =
-  let doc = "TODO" in
-  let docv = "ICAL" in
-  Arg.(required & pos ~rev:true 0 (some string) None & info [] ~docv ~doc)
+let files =
+  let doc = "Files to process" in
+  Arg.(non_empty & pos_all string [] & info [] ~docv:"FILE" ~doc)
 
 let main_command f =
   let doc = "Convert iCalendar files to remind format" in
   let man = [] in
   Cmd.make (Cmd.info "ical2rem" ~version:"%%VERSION%%" ~doc ~man)
   @@
-  let+ ical_file = ical_file in
-  f ical_file
+  let+ files = files in
+  f files
 
 let main f = Cmd.eval @@ main_command f