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/CLAUDE.md

@@ -195,9 +195,8 @@ Standalone local-image lines (`![alt](./image.png)`) are auto-uploaded to Notion
 
 **Requirements:**
 - `ntn` CLI installed: `curl -fsSL https://ntn.dev | bash`
-- Logged in: `ntn login`
 
-Uploads are workspace-scoped — the file lands in whichever workspace the `ntn` CLI is authenticated to. Ensure this matches the workspace the Notion API token targets, or the image block will not attach.
+Uploads run as the **same integration** that writes the page (`NOTION_API_KEY`). The script injects `NOTION_API_TOKEN=$NOTION_API_KEY` into every `ntn` subprocess, so the file upload and the page share one identity — no separate `ntn login` or workspace matching is needed. `ntn` only needs to be installed, not logged in.
 
 ### Engines