docs(adr): ADR 0024, LanguageTool as a sidecar

The developer's decisions of 26 Oct (D1-D5) and the calls made
building it; notes in ADR 0017 (point 10) and ADR 0021 (D54) on what
it amends; Learn's AGENTS.md on the sidecar and the checks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

docs/adr/0021-private-instance-and-assistant.md

@@ -9,6 +9,10 @@
   the developer amended on 26 Oct to what was built.
 - **Builds on:** ADR 0016 (the register), ADR 0019 (private studysets), ADR 0020
   (SimpleLogin), ADR 0022 (the hand-off and the connector gone).
+- **Amended by:** [ADR 0024](0024-languagetool-sidecar.md), D3 (26 Oct 2026): the field's
+  Check (D54) goes to the instance's LanguageTool first where it checks the language,
+  whose sheet offers to ask the assistant why; elsewhere it asks the assistant, as D54
+  has it.
 - **Changes:** the rule of 10 Oct 2026 that Halkyon calls no language model with an API
   key, for an instance whose operator opts in.