test(e2e): isolate XDG_DATA_HOME from CAAM_HOME in rotation workflow test

The migrator now refuses to migrate when CAAM_HOME data path lives inside the
legacy XDG_DATA_HOME/caam tree, surfacing as "data migration skipped: refusing
to migrate" noise in test output. Use a disjoint xdg-data subdir so the
rotation flow exercises the intended migration-clean state.

Co-Authored-By: Claude <noreply@anthropic.com>