ipx add --list or --category updates a feed the catalogue already has (#107)

It refused one already there ("already subscribed as ..."), so a feed added before listing
existed, such as CBC's, could not be put in the Directory.

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

CHANGELOG.md

@@ -10,7 +10,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - `ipx add --list --category News <url>` puts a feed in the Directory for anyone to subscribe to,
-  and it stays there when its last subscriber leaves.
+  and it stays there when its last subscriber leaves. Run for a feed already in the catalogue,
+  it lists it or sets its category.
 - Feeds nobody subscribes to that are dead for a month or quiet for a year are removed, so the
   Directory lists feeds worth taking.