I’d use:
<li>
<blockquote cite="original URL">
</blockquote>
</li>
<blockquote> is most appropriate for this:
The blockquote element represents a section that is quoted from another source.
Quoting yourself is not weird. The definition of the element doesn’t say you have to quote somebody else. You do quote a document that lives under another URL.
I think <article> (instead of <li>) would be acceptable, but it’s not necessary (the definition of article is pretty lax, it should have been called <infolump> 😉
My litmus test for <article> is whether it’d be useful in an RSS/Atom feed, and you can find feeds with just article excerpts.
The <summary> element is only a part of <details>, which has different purpose.
<aside> describes role of the content within a page. When the excerpts are the main part of archive page, it’s not really a tangential content. OTOH if it were a “See also” section on a post page, then <aside> would be perfect.