feat(KB-648): enable parallel test execution and optimize test performance

- Enable parallel file execution in vitest configs for core, engine, CLI, and dashboard packages
- Optimize backup tests with fake timers for faster, deterministic execution
- Fix SettingsModal temporal dead zone by moving declaration before useCallback
- Fix engine tests with missing git branch delete mock and correct branch names
- Fix git worktree list mock and mission store mock in CLI tests
- Add changeset documenting test optimization patterns
- Update AGENTS.md with test optimization best practices

packages/dashboard/app/components/SettingsModal.tsx

@@ -79,6 +79,9 @@ export function SettingsModal({
   const [activeSection, setActiveSection] = useState<SectionId>(initialSection ?? SETTINGS_SECTIONS[0].id);
   const [prefixError, setPrefixError] = useState<string | null>(null);
 
+  /** Get the scope of the currently active section */
+  const activeSectionScope = SETTINGS_SECTIONS.find((s) => s.id === activeSection)?.scope;
+
   // Auth state (independent of the settings save flow)
   const [authProviders, setAuthProviders] = useState<AuthProvider[]>([]);
   const [authLoading, setAuthLoading] = useState(false);
@@ -339,9 +342,6 @@ export function SettingsModal({
     [onClose],
   );
 
-  /** Get the scope of the currently active section */
-  const activeSectionScope = SETTINGS_SECTIONS.find((s) => s.id === activeSection)?.scope;
-
   const handleSave = useCallback(async () => {
     if (prefixError || presetDraft) return;
     try {