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

[hanthor]

Triage note — still valid and still unlanded. Leaving open.

I verified the invariant this test guards against current main:

• SKIP_DOCS_DIR (declared at scripts/sync-org-docs.mjs:76) does contain tunaOS, and its comment explicitly records the removed docs/book/src mapping
• DOCS_SUBDIR is {} at line 88, so DOCS_SUBDIR.tunaOS is undefined

So both assertions would pass today — the invariant holds, but nothing guards it. That is exactly the case this PR is for: the mapping was removed once already, and there is currently no test stopping it from being silently revived and overwriting the hand-authored docs/tunaos tree.

The diff is small (exporting SKIP_DOCS_DIR/DOCS_SUBDIR plus one test) and test-only. It needs a rebase onto current main and to be taken out of draft; I have not done that here because the PR head is on a fork this session cannot push to.

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