fix: allow PR worktrees from existing local branches

Use existing local PR branches instead of blocking worktree creation
Only block PR branches already checked out in another worktree
Clarify local branch validation messages across locales

packages/ui/src/lib/i18n/messages/zh-CN.ts

@@ -1289,6 +1289,7 @@ export const dict: Record<I18nKey, string> = {
   'session.githubIntegration.error.notConnected': 'GitHub 未连接',
   'session.githubIntegration.error.loadDataFailed': '加载数据失败',
   'session.githubIntegration.validation.branchAlreadyCheckedOut': '该分支已在某个 worktree 中检出',
+  'session.githubIntegration.validation.branchAlreadyExists': '该分支已在本地存在',
   'session.githubIntegration.validation.failed': '校验失败',
   'chat.fileAttachment.toast.attachFailed': '附加文件失败',
   'chat.fileAttachment.toast.someFilesSkipped': '以下文件被跳过：\n{summary}',