boinger / confvis / 24679267795

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fix(history): PruneRef uses compare-and-swap to preserve concurrent appends

PruneRef was written with unconditional WriteToRef, meaning a
concurrent AppendToRef between PruneRef's read and write would
silently lose the appended entry. AppendToRef itself correctly uses
CAS via gitutil.WriteRef(ref, content, oldSHA), so this was a
convention mismatch.

Match the AppendToRef pattern: read oldSHA first, pass through to
WriteRef, propagate ErrRefConflict to the caller. Update the caller
in appendToHistory to unwrap ErrRefConflict as benign -- the append
already succeeded, and the next prune run will see the new state.

New tests: TestGitRefStorage_PruneRef_Conflict exercises the CAS
failure path and asserts the concurrent entry is preserved.
TestGitRefStorage_PruneRef_HappyPath covers the no-conflict case.
All 10 pre-existing PruneRef tests continue to pass unchanged.

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