Monorepo preparation does not include repositories required by downstream mission-review gates

Observed while preparing the #2658 workspace (TF-052).

## Reproduction

1. Use the supported monorepo-preparation workflow for a Spec Kitty feature and select a workflow that will later run mission review.
2. Observe only the main `spec-kitty` repository is prepared.
3. Reach the mandatory cross-repository E2E gate.
4. Discover `spec-kitty-end-to-end-testing` is absent and must be cloned/configured manually.

## Expected

Preparation should resolve the repository set from selected downstream skills/gates, or preflight should declare the missing sibling before work begins and offer a deterministic add-repository step.

Start with the supported monorepo-preparation workflow and trace how it resolves repositories and downstream skills or gates. Reproduce the #2658/TF-052 scenario through the mandatory cross-repository E2E gate; done means the required sibling is prepared automatically or preflight identifies it and provides a deterministic add-repository step.

Written by the indexing model from the issue text.