[codex] externalize course sources from builder repo

[puppe1990]

What changed

• removed tracked course source content from the repository
• made the builder load courses from an external source directory and sync them into docs/courses/ for local build/runtime
• updated the multi-course registry, catalog, scripts, docs, and tests for the external-source model
• ignored local/generated course artifacts so the repo stays builder-only

Why

The repository should remain a reusable course builder, not the storage location for course content. Externalizing course sources keeps the repo focused on platform code while preserving the multi-course build behavior.

Validation

• npm test
• npm run docs:build

Notes

• default external source directory is ../course-builder-courses
• COURSE_SOURCE_DIR can override the source path locally