feat(history): 消息正向增量拉取(A5) — GET /sessions/{id}/messages?after=

- history/list_after.go: ListAfter(升序, created_at>after, 走覆盖索引, rowid兜底同毫秒)
- HistoryStore 新增 ListAfter(store+fakes同步)
- gateway history.go: handleListMessages解析after分流; after+before互斥400; 负数/非数字400
- 新增测试(list_after/history_after), build/vet/test绿+交叉编译
- Docs/api-client v0.26 + api-reference §3.4 同步

internal/services/gateway/session_orchestrator_test.go

@@ -179,6 +179,10 @@ func (f *fakeHistory) List(context.Context, string, int64, int, string) ([]histo
 	return nil, nil
 }
 
+func (f *fakeHistory) ListAfter(context.Context, string, int64, int, string) ([]history.Message, error) {
+	return nil, nil
+}
+
 func (f *fakeHistory) Recent(context.Context, string, int) ([]history.Message, error) {
 	return nil, nil
 }