refactor(session): return RetentionResult instead of bare tuple

Replace the tuple(list[dict], int) return of
Session.retain_recent_legal_suffix with a named RetentionResult
dataclass that exposes retained, dropped,
already_consolidated_count, and new_last_consolidated fields.

The tuple return was easy to misuse because the second value only
made sense relative to the first and the old last_consolidated
cursor. The named fields make the archive-skip semantics explicit
at every call site.

No behavior change. All existing tests pass unchanged in semantics.

Refs #4136

Signed-off-by: axelray-dev <110029405+axelray-dev@users.noreply.github.com>

nanobot/agent/memory.py

@@ -1018,9 +1018,9 @@ class Consolidator:
                 metadata={},
                 last_consolidated=0,
             )
-            dropped, already_consolidated = probe.retain_recent_legal_suffix(max_suffix, extend_to_user=True)
+            result = probe.retain_recent_legal_suffix(max_suffix, extend_to_user=True)
             messages_to_keep = probe.messages
-            messages_to_remove = dropped[already_consolidated:]
+            messages_to_remove = result.dropped[result.already_consolidated_count:]
 
             if not messages_to_remove and not messages_to_keep:
                 session.updated_at = datetime.now()