GitHub list responses include navigation headers

Date: 2026-09-29. Card: t_974d735b, task t_49ea9a88.

GitHub's manifest declares numbered pagination, but its custom list handlers only sliced rows. SDKs following Link could stop at the first page even when more matching rows existed.

The GitHub pack renders navigation from the filtered collection's count and the same page parameters used to slice it. Handlers retain their wire shapes, including envelopes and total counts. Links preserve the request address and filters. The pack shares this rendering across its custom handlers; the kernel's existing manifest pagination remains unchanged.

The behavior follows GitHub's pagination documentation. This decision covers the existing numbered list handlers. Cursor-based endpoints retain their own vendor-specific behavior.