[codex] Ensure but-graph keeps exact target segments

[Byron]

Summary

• ensure but-graph always creates an exact target segment for configured workspace target refs when the graph would otherwise only contain the local tracking segment
• keep workspace target resolution stable in ambiguous remote/local shapes, including cases where the remote ref itself is absent but the local branch still tracks it
• update the existing ambiguous-with-remotes snapshots and add a focused regression for the missing target-segment case

Root Cause

but-graph could end up with only a local tracking segment like main <> origin/main and no exact refs/remotes/origin/main segment. The workspace projection expects the configured target ref to exist as its own segment, so target resolution could fail or become shape-dependent after post-processing.

Impact

This normalizes the graph shape for configured workspace targets so downstream workspace logic can rely on the exact target segment existing when possible.

Validation

• cargo fmt --check --all
• cargo test -p but-graph

Notes

• This draft PR was created by Codex.
• Byron still needs to review the change before it should be merged.

[vercel]

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

A member of the Team first needs to authorize it.