fix: propagate real error when machineBash fails in createWorktree

createWorktree unconditionally reported "Not a Git repository" for any
machineBash failure, hiding RPC errors and network issues. Now checks
exitCode === -1 (RPC/communication failure) and forwards the real error.

Fixes #1110

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: yhxiang

packages/happy-app/sources/utils/worktree.ts

@@ -55,7 +55,9 @@ export async function createWorktree(
             success: false,
             worktreePath: '',
             branchName: '',
-            error: 'Not a Git repository'
+            error: gitCheck.exitCode === -1
+                ? (gitCheck.stderr || 'Failed to reach machine')
+                : 'Not a Git repository'
         };
     }