Report has_more on cursor-opposite side when cursor is set

Paging backward with --before TS proves messages newer than TS exist
(TS itself is one of them), so has_more_after must be true. The old
logic only derived has_more from peek-ahead detection and missed the
cursor signal, leaving users at the end of a --before page with no way
to turn around via the footer or JSON cursors. Same symmetry for
--after TS implying has_more_before. Applied to both get_messages and
get_thread_replies.

Author: fprochazka

src/slackcli/client.py

@@ -272,6 +272,14 @@ def _ts_to_float(value: str) -> float:
             elif api_has_more_after_fetch:
                 has_more_after = True
 
+        # Cursor-implied has_more: the caller's own cursor proves there is
+        # content on the opposite side of the returned slice, even when the
+        # API's peek-ahead / has_more signal does not reach that far.
+        if before_ts is not None:
+            has_more_after = True
+        if after_ts is not None:
+            has_more_before = True
+
         return messages, has_more_before, has_more_after
 
     def get_thread_replies(
@@ -385,6 +393,13 @@ def _ts_to_float(value: str) -> float:
                 filtered = filtered[:count]
                 has_more_after = True
 
+        # Cursor-implied has_more: the caller's own cursor proves there is
+        # content on the opposite side of the returned slice.
+        if before_ts is not None:
+            has_more_after = True
+        if after_ts is not None:
+            has_more_before = True
+
         return [parent, *filtered], has_more_before, has_more_after
 
     def fetch_full_thread(