📝 Prune the backlog; document the LSP shutdown fix

Remove every done/cancelled backlog item; the reasons they recorded are
already in development/ (library.md, testing.md, tooling.md, workflow.md)
and git history is the archive.

The one exception was the TS 7 LSP shutdown workaround, which lived only in
a code comment. Record it under development/testing.md § Autocomplete
(Known issue) before dropping the bug item.

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- prune the backlog of completed items and record the TS 7 LSP shutdown
+  workaround in development/testing.md
+
 ## [0.7.1] - 2026-09-23
 
 - upgrade dependencies