[codex] Fix export paths for active course docs

[puppe1990]

What changed

• switched export-site content discovery to resolve from the active course docs root instead of hard-coding docs/<locale>
• updated README screenshot export to write into the active course public directory and mirror into the legacy docs/public/screenshots/readme path
• added a regression test that asserts the active course docs tree resolves to real content

Why

VitePress now reads markdown from courses/<slug>/docs via ACTIVE_COURSE_CONTENT_DIR, but the export pipeline was still resolving source content from docs/. That broke PDF export after the per-course docs migration because docs/en no longer exists.

Impact

• pdf:export follows the active course content tree
• README screenshot generation stays compatible with the current README asset links
• the regression is covered by tests

Validation

• npm test
• npm run docs:build