📝 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.

development/testing.md

@@ -198,6 +198,11 @@ the CLI is for manual inspection.
   delay is needed.
 - The server answers some requests with a string id (`client/registerCapability`);
   the client must tolerate `string | number` ids or the server stalls.
+- `LspSession.close()` sends `shutdown` and then closes stdin instead of
+  sending `exit`. The TS 7 Go server's `handleExit` returns `io.EOF`, cancelling
+  the background context while a watch update is still in flight, and logs a
+  bare `context canceled` before exiting 1; EOF on stdin exits 0 with no output.
+  The kill stays as a fallback for a server that does not exit.
 
 ## Known issues