When spawn() receives an explicit cwd (errands), the manager was still
creating a standalone worktree at agent-workdirs/<alias>/ and injecting
its path into the workspace layout prompt. The agent then edited files
in the wrong directory — on a different branch than the errand's.
Now when cwd is provided, we skip worktree creation entirely and use
the caller's cwd for workspace layout, .cw/output/, and all paths.
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