docs: add auto-update implementation plan and rescope roadmap

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/CHANGELOG.md

@@ -16,6 +16,9 @@ restructured around what actually blocks each item.**
 
 **Documentation:**
 
+- **`AUTO_UPDATE_PLAN.md`** — design-first plan for GitHub Release checks,
+  signed portable installs, and in-app restart. **`ROADMAP.md`** retitled the
+  feature to auto-update and points at the plan instead of detection-only scope.
 - **`ROADMAP.md`** — reorganized into Features, Platform, Fyne/UI, and
   Maintenance. The over-the-guideline file-split item is closed; each deferred
   UI item now states whether it is blocked on a missing Fyne API, on UX