Handle empty repositories when remote HEAD/default branch is unknown

[sven1103-agent]

Summary

worktree_prepare and auto-provisioned write flows can fail in repositories that have no commits yet.

In that state, remote/default-branch detection can resolve to "(unknown)", and the plugin later tries to fetch or use that as a real ref, which produces:

• fatal: couldn't find remote ref (unknown)

Current behavior

When the repository is still empty and no branch exists remotely yet, default-branch detection may surface (unknown) from git remote show <remote>.

The plugin currently treats that as a valid branch name and continues into base-target resolution, which then tries to fetch or use (unknown) as the base ref.

Expected behavior

Empty repositories should be handled explicitly.

Acceptable outcomes include:

• detect the empty or unborn repo state and return a clear, actionable error
• or skip remote base resolution until an initial commit or default branch exists
• but do not treat (unknown) as a valid branch or ref

Why this matters

This blocks worktree provisioning and contributes to broken plugin-managed write flows during repository bootstrap.

Related

• Related to Plugin loads, but write flow is not redirected into prepared worktree #62
• This is a distinct empty-repository edge case that should be fixed independently of the broader runtime write-redirection problem.