fix(ntn-files): propagate NOTION_API_KEY into ntn subprocesses

Force ntn to authenticate as the same integration that writes the page by
injecting NOTION_API_TOKEN=$NOTION_API_KEY into every ntn subprocess env.
Notion scopes file uploads to the creating integration, so a mismatch caused
"Could not find file_upload with ID …" when attaching uploaded images.

Added test_upload_passes_token_env (9 total in test_ntn_files.py) and updated
code/CLAUDE.md to reflect that a separate ntn login is no longer required.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

custom-skills/32-notion-writer/code/scripts/test_ntn_files.py

@@ -116,6 +116,18 @@ def test_materialize_missing_file_raises():
         assert "nope.png" in str(e) or "nope.png" in e.path
 
 
+def test_upload_passes_token_env():
+    _reset_cache()
+    fake = subprocess.CompletedProcess(args=[], returncode=0,
+        stdout="ID123\tphoto.png\tuploaded\n", stderr="")
+    with mock.patch.dict("os.environ", {"NOTION_API_KEY": "tok-abc"}, clear=False), \
+         mock.patch("builtins.open", mock.mock_open(read_data=b"x")), \
+         mock.patch("subprocess.run", return_value=fake) as m:
+        ntn_files.upload(Path("/tmp/p.png"))
+        passed_env = m.call_args.kwargs["env"]
+        assert passed_env["NOTION_API_TOKEN"] == "tok-abc"
+
+
 def run_all():
     tests = [
         test_preflight_missing_ntn,
@@ -126,6 +138,7 @@ def run_all():
         test_materialize_local_uploaded,
         test_materialize_nested_children,
         test_materialize_missing_file_raises,
+        test_upload_passes_token_env,
     ]
     for t in tests:
         print(f"\n{t.__name__}")