server: reject a non-repo turn diff before any turn

workspace.diff with scope "turn" returned an empty turn:null summary for a workspace that is not a Git repo; docs/08-ipc.md promises BAD_PARAMS for a non-repo, as the HEAD scope does.
71c132e17fGrok Bot committed on 10/10/2026, 4:20:35 PMparent85ef76c

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