feat(cli): add output control flags, sort order, and verbose mode

- Add `--verbose`/`-v` flag; gate all diagnostic stderr output behind it
- Add `--no-uuid`, `--no-source`, `--no-location`, `--no-description`,
  `--no-conference-url` flags to suppress individual INFO lines
- Add `--sort` option (`asc`, `desc`, `original`) replacing hardcoded
  descending sort
- Add `--source` option to override calendar name (single-file only)
- Introduce `Config` module with global `ref` flags set at startup from
  CLI args
- Add `Utils.warn` helper that writes to stderr only when
  `Config.verbose` is set
- Normalise all diagnostic messages to a consistent format (`Warning:
  ... (UID: ...)`)
- Remove `debug_print_of_recurrence_and_skip`; inline skip at each call
  site
- Fix `add_common_part` to always emit a trailing `\\\n    `
  continuation line

bin/config.ml

@@ -0,0 +1,10 @@
+(** Global configuration flags, set once at startup from CLI args. *)
+
+(** Enable diagnostic output on stderr. Off by default; activated by --verbose. *)
+let verbose = ref false
+
+let no_uuid = ref false
+let no_source = ref false
+let no_location = ref false
+let no_description = ref false
+let no_conference_url = ref false