Revert "refactor: clean up legacy code, rename hopper to entity_extractor, and add third-party notices"

This reverts commit 54fcef9887.

Please do not submit unrelated docs and codes in the same commit. I am reverting this commit so that the related files can be submitted in better-organized future commits.

artemis/utils/time.py

@@ -0,0 +1,20 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from datetime import datetime
+
+
+def convert_timestamp_to_str(ts: float) -> str:
+    dt = datetime.fromtimestamp(ts)
+    return dt.strftime("%Y-%m-%dT%H-%M-%S")