Adds newsletter infrastructure: CLI commands (new, list, preview, publish,
fetch, status) integrated into binder, Quarto archive site config for
mlsysbook.ai/newsletter/, and 12-month editorial content plan. Drafts
are gitignored for private local writing; sent newsletters are committed
as the public archive.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
