docs: rewrite README, add user guide, sync CONFIG.md with code

- README.md: 324 -> 106 lines, clickable TOC, quick start, formats
  table, full check suite + CI; flags/env/YAML reference moved to docs
- docs/USER_GUIDE.md: new user guide (install, periods, users, export,
  email, monthly --commit cycle, 21-flag reference, troubleshooting)
- docs/CONFIG.md: clickable TOC + accuracy fixes verified against code
  (dedup logic was described backwards, missing report section in
  --init-config, conditional STARTTLS, full --config-path role,
  datetime --date ranges, --config override nuance, comment loss on
  --commit, default_to without default_from is ignored)
- client.py: dedup docstring/comment now match actual behavior

redmine_reporter/client.py

@@ -275,8 +275,11 @@ def fetch_issues_with_spent_time(
     along with total spent hours per issue.
     If user_id is None, uses current user.
     If by_activity is True, returns per-activity breakdown as third tuple element.
-    If dedup_before is set, filters out time entries whose created_on AND updated_on
-    are both before dedup_before (AND logic: both must be < cutoff to exclude).
+    If dedup_before is set, entries with both fields set are kept only when
+    created_on AND updated_on are both >= dedup_before; an entry is excluded
+    if at least one of the fields is before the cutoff (dedup_before).
+    Entries with both fields missing (None) are kept;
+    if only one field is set, that field alone decides (>= cutoff keeps the entry).
     Returns list of (issue, total_hours, activities) tuples.
     Raises RedmineAPIError on API/auth/network failures.
     """
@@ -300,8 +303,11 @@ def fetch_issues_with_spent_time(
         raise RedmineAPIError(_format_redmine_error(exc), original=exc) from exc
 
     # Дедупликация: отсекаем записи, которые были учтены в предыдущем отчёте.
-    # Запись исключается, если BOTH created_on AND updated_on < dedup_before.
-    # Записи без метаданных (created_on/updated_on == None) не фильтруются.
+    # Если оба поля заданы, запись сохраняется, только если created_on И updated_on
+    # оба >= dedup_before; если хотя бы одно из полей < dedup_before,
+    # запись исключается.
+    # Если оба поля None — запись сохраняется; если задано только одно поле,
+    # решает оно (>= dedup_before → запись сохраняется).
     if dedup_before is not None:
         # Нормализуем cutoff к aware UTC (#58): naive cutoff трактуем как UTC,
         # чтобы сравнение с нормализованными created_on/updated_on было корректным.