A pull's Files tab with a line comment has no sideways scroll at 390px or desktop, with looks on the pull #10097

The pull's preview now takes desk and phone looks of its Files tab with a review comment on a diff line. I couldn't check the sideways-scroll claim by measurement here, because no Chromium starts in this sandbox.

• Seed: src/spawn/preview-seed.ts posts one line comment on GREETING.md line 1 of the seeded pull. It is plain prose with an 80-character unbroken word as the worst case. The comment uses the existing addReview action, not a new path.
• Looks: PREVIEW_PULL_FILES in src/spawn/preview-looks.ts joins PREVIEW_OWNER_LOOKS, so the pull's preview takes desktop and phone looks of /files.
• Overflow: I found nothing overflowing, so I changed no page code and added no class. The comment body is .prose, which wraps anywhere, and the diff scrolls inside its own wrapper. This is by reading the CSS only. I noted it in docs/design/areas/pull.md.
• Tests: test/pull-phone-fit.test.ts now checks that the Files tab shows the comment and that both looks are listed. It also has 390px and 1280px scrollWidth checks, which skip without a Chromium.
• Checks:

- bun x tsc --noEmit and bun scripts/design-lint.ts pass. - bun scripts/gate.ts printed nothing, so I take it as clean. - test/pull-phone-fit.test.ts and test/looks-owner.test.ts pass, with the 4 Chromium-dependent tests skipped. - I did not run test/preview-seed*.test.ts; that glob matched no file. - I did not run the full bun test; the forge runs it on push.

• How to look: open the pull's preview, then /preview-admin/sample/pulls/3/files. The two looks appear on the pull once the forge takes them.
• Not done: I didn't call keep_plan because that tool isn't available here.

Closes #10073

The pull's preview now takes desk and phone looks of its Files tab with a review comment on a diff line. I couldn't check the sideways-scroll claim by measurement here, because no Chromium starts in this sandbox.

• Seed: src/spawn/preview-seed.ts posts one line comment on GREETING.md line 1 of the seeded pull. It is plain prose with an 80-character unbroken word as the worst case. The comment uses the existing addReview action, not a new path.
• Looks: PREVIEW_PULL_FILES in src/spawn/preview-looks.ts joins PREVIEW_OWNER_LOOKS, so the pull's preview takes desktop and phone looks of /files.
• Overflow: I found nothing overflowing, so I changed no page code and added no class. The comment body is .prose, which wraps anywhere, and the diff scrolls inside its own wrapper. This is by reading the CSS only. I noted it in docs/design/areas/pull.md.
• Tests: test/pull-phone-fit.test.ts now checks that the Files tab shows the comment and that both looks are listed. It also has 390px and 1280px scrollWidth checks, which skip without a Chromium.
• Checks:

- bun x tsc --noEmit and bun scripts/design-lint.ts pass. - bun scripts/gate.ts printed nothing, so I take it as clean. - test/pull-phone-fit.test.ts and test/looks-owner.test.ts pass, with the 4 Chromium-dependent tests skipped. - I did not run test/preview-seed*.test.ts; that glob matched no file. - I did not run the full bun test; the forge runs it on push.

• How to look: open the pull's preview, then /preview-admin/sample/pulls/3/files. The two looks appear on the pull once the forge takes them.
• Not done: I didn't call keep_plan because that tool isn't available here.

Left #10073 open: 1 unticked item remain (The pull carries looks of a pull's Files tab with a review comment on a diff line, at desktop width and at 390px.).