fix(session): persist auto-reset state across gateway restarts

was_auto_reset, auto_reset_reason, and reset_had_activity were not
included in SessionEntry.to_dict() / from_dict(), so a gateway restart
between session expiry and the user's next message would silently drop
the auto-reset notification and context note.

Add the three fields to the serialization roundtrip with safe defaults
(False / None / False) so existing sessions.json files load cleanly.

Add three roundtrip tests to test_session_reset_notify.py.

gateway/session.py

@@ -518,6 +518,9 @@ class SessionEntry:
                 else None
             ),
             "is_fresh_reset": self.is_fresh_reset,
+            "was_auto_reset": self.was_auto_reset,
+            "auto_reset_reason": self.auto_reset_reason,
+            "reset_had_activity": self.reset_had_activity,
         }
         if self.origin:
             result["origin"] = self.origin.to_dict()
@@ -567,6 +570,9 @@ class SessionEntry:
             resume_reason=data.get("resume_reason"),
             last_resume_marked_at=last_resume_marked_at,
             is_fresh_reset=data.get("is_fresh_reset", False),
+            was_auto_reset=data.get("was_auto_reset", False),
+            auto_reset_reason=data.get("auto_reset_reason"),
+            reset_had_activity=data.get("reset_had_activity", False),
         )