[codex] Improve error for refs outside workspace

[Byron]

Summary

• improve create_reference so an existing ref outside the workspace fails with an explicit error instead of the generic segment-creation error
• add a regression test covering that exact case

Why

The reported failure mode in issue #13104 surfaced a cryptic error when the branch already existed in Git but was outside the GitButler workspace.

Validation

• cargo test -p but-workspace create_reference -- --nocapture
• cargo fmt --check --all

Notes

• This draft PR was created by Codex.
• Byron still needs to review this change before it is ready to merge.

[vercel]

@Byron is attempting to deploy a commit to the GitButler Team on Vercel.

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