chore: import upstream snapshot with attribution

scripts/check_new_py_files.sh

@@ -0,0 +1,52 @@
+#!/bin/bash
+# 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.
+
+
+exit_code=0
+
+get_added_files() {
+  if git rev-parse --is-inside-work-tree >/dev/null 2>&1; then
+    git diff --cached --name-only --diff-filter=A
+  elif jj root >/dev/null 2>&1; then
+    jj diff --summary 2>/dev/null | awk '/^A / {print $2}'
+  elif hg root >/dev/null 2>&1; then
+    hg status --added --no-status 2>/dev/null
+  elif g4 info >/dev/null 2>&1; then
+    g4 opened 2>/dev/null | awk '/ - add / {print $1}' | sed 's/#.*//'
+  elif p4 info >/dev/null 2>&1; then
+    p4 opened 2>/dev/null | awk '/ - add / {print $1}' | sed 's/#.*//'
+  fi
+}
+
+while read -r file; do
+    # Check if file is not empty (happens if no new files)
+    if [[ -n "$file" ]]; then
+        # Match only files in the package source (src/google/adk/) to avoid false
+        # positives in environments (e.g., monorepos) where the entire repository
+        # root is nested under a 'google/adk/' directory structure.
+        if [[ "$file" == */src/google/adk/*.py ]] || [[ "$file" == src/google/adk/*.py ]]; then
+            filename=$(basename "$file")
+            if [[ ! "$filename" == _* ]]; then
+                echo "Error: New Python file '$file' must have a '_' prefix."
+                echo "All new Python files in src/google/adk/ must be private by default."
+                echo "To expose a public interface, use __init__.py and list public symbols in __all__."
+                echo "See .agents/skills/adk-style/references/visibility.md for details."
+                exit_code=1
+            fi
+        fi
+    fi
+done < <(get_added_files)
+
+exit $exit_code