feat(workflow): configure required branch approvals

Plan

1. Validate the optional required count and pass it through branch-rule configuration to GitLab approval rules.
2. Cover default and explicit counts, single-rule compatibility, and both workflow components in tests and documentation.

Acceptance criteria

• Omitted or empty approvers selects project Maintainers and Owners with zero required approvals unless required is set.

• Non-empty approvers defaults to one required approval; an explicit non-negative integer overrides either default.

• Invalid required values fail validation, and a visible single-rule fallback must match the requested count exactly.

• Workflow tests pass locally and both component guides describe the behavior.

• The exact-head MR pipeline succeeds.

• The final diff and rebase are reviewed.

Exact-head pipeline passed all required jobs.

Closes #407 (closed)