build(editor): halkyon_editor joins the workspace (ADR 0033, D37)

The package for the general parts of Learn's editor, its strings first;
the code moves next.

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

pubspec.yaml

@@ -28,6 +28,7 @@ workspace:
   - packages/halkyon_base_client
   - packages/halkyon_base_server
   - packages/halkyon_core
+  - packages/halkyon_editor
   - packages/halkyon_interop
   - packages/halkyon_keys
   - packages/halkyon_llm