feat(KB-184): change autoMerge default from false to true

- Update DEFAULT_SETTINGS in core types to set autoMerge: true
- Update initial state defaults in App and SettingsModal components
- Update test fixtures in App, Board, and SettingsModal tests
- Update kb-board skill docs to reflect new default
- Add minor changeset for the default change

packages/core/src/types.ts

@@ -159,7 +159,7 @@ export const DEFAULT_SETTINGS: Settings = {
   maxWorktrees: 4,
   pollIntervalMs: 15000,
   groupOverlappingFiles: false,
-  autoMerge: false,
+  autoMerge: true,
   worktreeInitCommand: undefined,
   recycleWorktrees: false,
   taskPrefix: undefined,