use_cases.md: a story for `campaign init` at a superstudy that already carries campaigns

`docs/spec.md` states the behavior (scaffold-or-adopt, the four init cases), so the code question is settled. What is missing is the user story: someone points `campaign init --superstudy` at a superstudy that already holds `c1` and `c2` and wants to know what happens to them. The answer is nothing: init never clobbers, and a campaign with the same label is refused by name.

Docs only, in `docs/use_cases.md`.

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Start with docs/spec.md to review the scaffold-or-adopt behavior and the four init cases. Add a user story to docs/use_cases.md covering campaign init --superstudy when c1 and c2 already exist, including that existing campaigns are not clobbered and duplicate labels are refused. Done means the scenario and outcome are documented clearly.

Written by the indexing model from the issue text.