Every preview of the forge shows the seeded pull page, repository head, home with pins and /flow, looked at as the owner #8865

Replaces #8861 and the open box on #8812. Since #8830 (902b0311) the forge itself takes desk and phone looks of every preview, signed in as the preview-only admin for the paths in forge.json app.looksAsOwner, over the fixture in src/spawn/preview-seed.ts. The builder does not take or attach looks: they show under Looks on the builder's own pull once its preview is up.

Done looks like:

• [x] forge.json app.looks and app.looksAsOwner add the seeded repository's home (/preview-admin/sample) and its seeded pull's page (/preview-admin/sample/pulls/<n>, whichever number the seed gives it). / and /flow stay listed.
• [x] A test boots the preview seed and renders each owner-look path as the preview admin. It finds pins on /, a pipe on /flow, the repo head's title on the repository home, and the Ship decision on the pull page, so a blank look fails the check rather than reaching review.
• [x] docs/design/areas/pull.md and docs/design/areas/repo-head.md each say in one line under their opening line that every forge pull's preview carries desk and phone looks of this page as the owner.
• [ ] The builder's pull lists, from its own preview's Looks, the desk and phone looks of these four pages.

Files: forge.json, src/spawn/preview-seed.ts, test/looks-owner.test.ts, docs/design/areas/pull.md, docs/design/areas/repo-head.md

Reviewed pull #8875 at its head: ticked forge.json app.looks and app.looksAsOwner add the seeded repository's home (/preview-admin/sample) and its seeded pull's page (/preview-admin/sample/pulls/<n>, whichever number the seed gives it). / and /flow stay listed.; A test boots the preview seed and renders each owner-look path as the preview admin. It finds pins on /, a pipe on /flow, the repo head's title on the repository home, and the Ship decision on the pull page, so a blank look fails the check rather than reaching review.; docs/design/areas/pull.md and docs/design/areas/repo-head.md each say in one line under their opening line that every forge pull's preview carries desk and phone looks of this page as the owner.. left open: The builder's pull lists, from its own preview's Looks, the desk and phone looks of these four pages. — The pull description does not list the desk and phone looks of the four pages; it says the forge will show them under Looks..

Left open after pull #8875 landed: 1 unticked item remains (The builder's pull lists, from its own preview's Looks, the desk and phone looks of these four pages.). The planner files each as its own issue and closes this one; a person may close it, tick what is done, or say what is left. Until then the crew does not pick it again.

Dropped: the seeded preview pages shipped. The open box asked the builder's pull to list its looks, which is evidence for that pull, not work for a new one. Any later preview's Looks show those pages.

Correction: this isn't dropped. A person's ask stands behind it, so it's replaced by #8974, which makes the four pages a tested list rather than evidence on one pull.