Leave lists that cannot move, and outdent code with Shift+Tab

Tab on a first list item, or one too deep to nest, no longer wraps it in an
empty item that only collaborators see; Shift+Tab on a top-level item moves
focus instead. In a code block Shift+Tab removes one leading tab or up to four
spaces, and Escape then Tab leaves, as a hint in the block header says.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>