The `remind_sync` library was acting as a thin re-export layer. This
commit removes it entirely and moves the only non-trivial utility
(`show_error` for `Timedesc.Date.Ymd.error`) directly into
`bin/utils.ml`. Dead `[@@deriving show]` annotations on `rem`,
`week_first_day`, and `error` types are also removed.
- Add `simple_weekly` type and `weekly` field to `rem` record
- Add `exdate` field to `rem` for excluded dates
- Add `collect_exdates` collector to pipeline
- implement weekly `RRULE` handling with `BYDAY`, `INTERVAL`,
`COUNT`/`UNTIL`
- Add `render_weekly` to emit one `REM` per weekday with `UNTIL`/`*N`
- Replace `timedesc_of_date_or_datetime` with
`timedesc_of_utc_or_timestamp_local` in utils
- Refactor `get_exdates`/`get_rdates` to separate dates, datetimes and
periods; add debug logging per UID
- Wrap reminder output in try/catch in main; drop trailing newline
duplication
- Mark implemented predicates (P00–P05, P09, P12, P14) with ✅;
remove P18–P20 (ignored/deferred)
- 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
