Function syncFolderToWebsiteNotesFolder

Syncs every markdown file in a folder and performs folder-scoped cleanup.

Edge cases handled:

• Exports nested markdown files recursively through Obsidian's folder tree.
• Removes stale indexed files only when their paths are inside the selected folder scope and were not overwritten by this sync.
• Keeps indexed files outside the selected folder intact.
• Merges newly exported referenced assets into the index, even when those assets live outside the folder cleanup scope.