Add refactoring plan document

Document a phased plan to restructure GoSentry into focused packages
under src/ (domain, storage, runner, scheduler, platform, app, ui) with
an application-service layer that owns state, and link it from the README.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -9,6 +9,7 @@ Project notes:
 - [Changelog](docs/CHANGELOG.md)
 - [Roadmap](docs/ROADMAP.md)
 - [Architecture](docs/ARCHITECTURE.md)
+- [Refactoring plan](docs/REFACTORING.md)
 
 ## Features