test: guard tunaOS sync source mapping

[hanthor]

Summary

• expose the tunaOS sync source configuration to the sync test suite
• add a regression test confirming the removed docs/book/src mapping stays absent
• verify tunaOS's docs/ tree remains protected from bulk sync

Context

Issue #145 identified that the old mdBook source path was removed while the site's docs/tunaos/ pages remain hand-authored. The current configuration correctly removes the stale mapping and skips the upstream docs/ directory; this test prevents that safety fix from regressing.

Validation

• node scripts/__tests__/sync-org-docs.test.mjs — 70 passed, 0 failed
• git diff --check