Adds `_step_numbers(*, has_marketplace, has_skills)` so step numbering
lives in one place instead of being split across three branches in
`resolve_lines`. Pins the unified layout in the tests:
No plugins: 1 install, 2 init, 3 catalog, 4 diagnose, 5 skills, 6 confirm
With plugins: 1, 2, 3, 4 preflight, 5 marketplace, 6 diagnose, 7 skills, 8 confirm
`agnes auth import-token` / `agnes auth whoami` are now banned from the
rendered prompt — `agnes init` subsumes them. The renamed
`test_resolve_lines_no_plugins_unified_six_step_layout` asserts those
strings are absent and that the new step headers (`Bootstrap your Agnes
workspace`, `Verify the data is queryable`) are present.
Plan: docs/superpowers/plans/2026-05-04-unified-setup-prompt.md task 2.
e16698c3cc