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/components/session/GitHubIntegrationDialog.tsx

@@ -195,13 +195,15 @@ export function GitHubIntegrationDialog({
         worktreeName: branchName,
       });
       
-      const isBlocked = result.errors.some(
-        (entry) => entry.code === 'branch_in_use' || entry.code === 'branch_exists'
-      );
+      const blockingError = result.errors.find((entry) => entry.code === 'branch_in_use');
       
       setValidations(prev => new Map(prev).set(branchName, {
-        isValid: !isBlocked,
-        error: isBlocked ? t('session.githubIntegration.validation.branchAlreadyCheckedOut') : null,
+        isValid: !blockingError,
+        error: blockingError
+          ? t(blockingError.code === 'branch_exists'
+            ? 'session.githubIntegration.validation.branchAlreadyExists'
+            : 'session.githubIntegration.validation.branchAlreadyCheckedOut')
+          : null,
       }));
     } catch {
       setValidations(prev => new Map(prev).set(branchName, {