📝 Prune the completed backlog items

Remove every done/cancelled item. Git history is the archive and each
item's rationale already lives in the README, in development/ or in a
released changelog entry; none was recorded only here, so no item had to
be rescued into development/ before it could go.

- drop the `v1.0:` and `Matcher:` projects too: their last open task is
  done, so those headers would stand empty
- summarise the change under `[Unreleased]` in the changelog

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- prune the completed items from the backlog; their rationale already lives in
+  the README, `development/` and the released changelog entries
+
 ## [0.8.2] - 2026-09-25
 
 - write the README's Synopsis and Examples sections