fix: harden install.sh against inherited Python env leakage

tests/test_install_sh_pythonpath_sanitization.py

@@ -0,0 +1,30 @@
+"""Regression tests for install.sh Python environment sanitization.
+
+When install.sh is launched from another Python-driven tool session, inherited
+PYTHONPATH/PYTHONHOME can shadow the freshly installed checkout. The installer
+must sanitize those vars both during installation and at runtime launch.
+"""
+
+from pathlib import Path
+
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+INSTALL_SH = REPO_ROOT / "scripts" / "install.sh"
+
+
+def test_install_script_unsets_pythonpath_and_pythonhome_early() -> None:
+    text = INSTALL_SH.read_text()
+
+    # During install, inherited Python env must be sanitized before pip/venv use.
+    assert 'unset PYTHONPATH' in text
+    assert 'unset PYTHONHOME' in text
+
+
+def test_hermes_launcher_wrapper_clears_python_env_before_exec() -> None:
+    text = INSTALL_SH.read_text()
+
+    # Wrapper should clear env and forward args untouched to the venv entrypoint.
+    assert 'cat > "$command_link_dir/hermes" <<EOF' in text
+    assert 'unset PYTHONPATH' in text
+    assert 'unset PYTHONHOME' in text
+    assert 'exec "$HERMES_BIN" "\\$@"' in text