Runners match ubuntu-latest's 16 GB in public repositories (needs a Cloudflare custom instance type) #190

GitHub's ubuntu-latest for public repositories has 16 GB of memory, and ours has 12 GiB (follow-up to #169). Matching it needs a Cloudflare Containers custom instance type (4 vCPU, 16 GiB) and a raised account limit.

1. Owner: ask Cloudflare for the custom instance type and the raised limit.
2. Once Cloudflare grants it, add a row to INSTANCE_TYPES (see CONTRIBUTING.md, "Adding a machine size"). Update pricing, the billing rate for the size, and the runner docs (machine sizes) to match.
3. Check it live: a public repository's job reports 16 GiB, and is billed as the machine it ran on.

This item is blocked until Cloudflare answers.

How to work on this repository

• Claim this item before you start (g1t work claim <number>), and add G1t-Session: $G1T_SESSION to every commit.
• Other sessions share the main checkout: work in a worktree of your own and stage only your own files.
• Pushing to main deploys production, and pushes to main and release tags are the owner's. Land the work through a pull request on this item, or commit locally and hand the owner the exact git -C <path> push ….
• Shipped means on main and deployed. Check it live with real data and look at the rendered pages: a status code doesn't show it works.
• Docs ship in the same change as the feature (guides, reference, llms.txt), to a high standard.
• User-facing text never names other products or compares g1t to them. Internal notes and code comments may.