Added a :::series remark plugin that allows blog posts to declare they belong to a series. When a post includes :::series[series-name], the plugin generates navigation showing all posts in the series with links to the others and “(this post)” for the current one.

Series are defined in src/data/series.ts with a title, ordered list of post slugs, and an optional footer HTML file for common content. Updated the “Steering the Vibe” posts to use this new directive instead of manually including the info box in each post.