docs: note SPARKLE_PRIVATE_KEY_FILE in profile

docs/reference/RELEASING.md

@@ -13,7 +13,7 @@ Use `pnpm` (Node 22+) from the repo root. Keep the working tree clean before tag
 ## Operator trigger
 When the operator says “release”, immediately do this preflight (no extra questions unless blocked):
 - Read this doc and `docs/platforms/mac/release.md`.
-- Load env from `~/.profile` and confirm `SPARKLE_PRIVATE_KEY_FILE` + App Store Connect vars are set.
+- Load env from `~/.profile` and confirm `SPARKLE_PRIVATE_KEY_FILE` + App Store Connect vars are set (SPARKLE_PRIVATE_KEY_FILE should live in `~/.profile`).
 - Use Sparkle keys from `~/Library/CloudStorage/Dropbox/Backup/Sparkle` if needed.
 
 1) **Version & metadata**