test(source-control): add a setup eval case that writes only what the user confirmed #7998

Refs: #7148

Agent Brief

Type: Task
Summary: One plugin-eval case shows source-control setup writes only the settings the user confirmed.

Current behavior:
plugins/source-control/evals has no setup case.

Desired behavior:
A case in the layout of the existing source-control cases runs setup in a scratch repository where the prompt confirms one change and declines another; deterministic graders check that only the confirmed change is written. No gh fixture is needed.

Key interfaces:

• The plugin-eval case layout.
• The setup skill's confirm-before-write contract.

Acceptance criteria:

• The case directory exists with case.yaml, prompt.md, scaffold and graders.
• A grader fails when the declined change appears in the workspace.
• A source-control changelog fragment is added and scripts/check-changelog-fragments.sh passes.

Out of scope:

• babysit cases and the gh stand-in.
• Paid runs.

This was generated by AI during backlog shaping. Split from #7148; the owner approved filing it in an attended backlog-shaping session on 2026-10-18.