[BUG] Agent subagents are forced to use isolation: "worktree" in non-Git directories, even with tengu_worktree_mode=false

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When using Agent subagents in Claude Code from a non-Git working directory, subagent creation repeatedly fails with: 
Error: Cannot create agent worktree: not in a git repository and no
WorktreeCreate hooks are configured. Configure
WorktreeCreate/WorktreeRemove hooks in settings.json to use worktree
isolation with other VCS systems.
In non-Git directories, subagent creation fails every time.

It appears Agent calls are still being forced through isolation: "worktree", which makes subagents unusable outside Git repos.

What Should Happen?

In non-Git directories, Agent should not be forced to use worktree isolation. If worktree is unavailable and not explicitly requested, Agent should gracefully fall back to non-worktree mode.

Error Messages/Logs

Error: Cannot create agent worktree: not in a git repository and no
     WorktreeCreate hooks are configured. Configure
     WorktreeCreate/WorktreeRemove hooks in settings.json to use worktree
     isolation with other VCS systems.

Steps to Reproduce

1. Start Claude Code in a non-Git directory (example: /Users/admin/app, not a git repo).
2. Trigger any request that causes an Agent subagent to be launched.
3. Observe that subagent creation fails with the worktree error.
4. Observe the same failure persists.

Claude Model

Other

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.71 (Claude Code)

Platform

Other

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_