Add option to disable automatic worktree creation in Desktop app

The Claude Code Desktop app automatically creates a new git worktree and branch for every conversation. There is no option to disable this behavior.

Current behavior

• Every new chat creates a worktree in .claude/worktrees/ and a branch claude/<random-name>
• "Worktree location" setting only offers "Inside project" and "Custom..." — no way to turn it off

Desired behavior

• Add a "Disabled" / "None" option so users can work directly on their current branch
• Worktrees should only be created when explicitly requested

Why

• Quick questions, debugging, small edits don't need branch isolation
• Accumulates stale branches and directories requiring manual cleanup
• CLI (claude) doesn't do this — desktop should match

macOS, version 1.2.234