Scaffold a vignette catalog

A catalog starts as a manifest and one useful notebook, not a package or pipeline. Ask for the catalog name, target path, and data surface (rest, duckdb, pooch, or files) only when they cannot be inferred.

Workflow

1. Inspect the target. For a new catalog, require an absent or empty directory. For an existing repository, preserve its files and use --adopt. Ask once which optional conventions to use, unless the user or existing repository policy already specifies the choices: standard data layout and working agreement, output summaries with an index notebook, and committed session snapshots. Explain the options in step 5; allow any combination or none, and wait for the answer before applying them.

2. Generate the minimum structure: