Pinning a post to the front page

The [c] action in ./blog.sh props <slug> pins a published post -- or unpins it again; pinned: true in the post's header (./blog.sh edit <slug>) does the same thing the long way round. A pinned post is held at the top of the front page, marked with a pin in the corner of its date badge. It appears there once: while it is still on the front page anyway it is lifted to the top rather than shown twice, and once it has aged onto /page/2/ the front page keeps the copy at the top while page 2 lists it in its normal place, unmarked.

Only the front page. Type and tag listings, the RSS feed, the sitemap and the search index stay strictly chronological -- a pin is a statement about the front page, not about the archive. Toggling it is a small deploy, because pagination is anchored and the front page is the only flexible one. Pin a second post and the newest of them wins, with a warning in the build output.