TINYDOC-3594 - Scope LLM file generation to the latest version's attachments

[kemister85]

Ticket: TINYDOC-3594 — follows TINYDOC-3592.

Problem: the generator found its output directory by searching the build for _attachments. A production build has one per version, so it wrote the version 8 corpus into version 6 and 7 as well — publishing four URLs carrying the wrong version's content, and leaving copy-llms-files.sh with six candidates instead of two, so /docs/llms.txt was set by find order.

Seen in the staging deploy, run 33481753889:

✓ Wrote build/site/tinymce/6/_attachments/llms.txt
✓ Wrote build/site/tinymce/7/_attachments/llms.txt
✓ Wrote build/site/tinymce/latest/_attachments/llms.txt

Fix: derive the path from BASE_URL rather than searching for it. parseSitemap() keeps only the URLs beneath BASE_URL, so the generator emits one version's corpus and must write to one version's directory. Falls back to the build root with a warning if that directory is absent.

Impact: staging only. Generation has never run against production, so no incorrect URL was published there.

Testing: full multi-version build via the production playbook — 1,582 pages, four versions, three _attachments directories.

• Writes only to tinymce/latest/_attachments; zero llms files under versions 5, 6 or 7.
• Copy step now finds two files, so the site root has one deterministic source.
• Site root, attachments copy and the files committed by TINYDOC: Regenerate llms.txt files to match latest build. #4331 are byte-identical.