Copy only the selected schemas with data copy --schema (S-007-1..5, S-007-14) #124

Builds part of spec S-007, docs/specs/007-data-copy-schemas.md (feature issue #82): the acceptance criteria below. Each needs a test that names its id (test_s007_<k>_..., or a proves: S-007-1 comment line).

This is data copy --schema: the selection, the receipt, the row counts, and the plan.

• S-007-1: Against a source with tables in schemas public, app, and audit, cloudfall data copy <service> --database <db> --source-url-file <file> --schema public --schema app --yes leaves the target with the tables of public and app only, and the receipt's spec.schemas is ["app", "public"]
• S-007-2: Against the same source, the copy without --schema leaves the target with the tables of all three schemas, as 0.7.0 does, and the receipt's spec.schemas is ["app", "audit", "public"]
• S-007-3: Each spec.tables[] entry of the receipt has schema, table, and rows, and the entries are exactly the base tables of the selected schemas
• S-007-4: The rendered row-count query for schemas app and public reads only those two schemas and returns one schema,table,rows line per base table, ordered by schema and table
• S-007-5: A target with a table in a selected schema refuses the copy with ENGINE_STEP_FAILED naming the table count, as today, and a target whose only table is in a schema not selected goes ahead
• S-007-14: cloudfall data copy without --yes names the selected schemas in its plan, or says it copies every non-system schema when no --schema is given