Design: a pull's merge box uses §5's card and buttons, and the merge-* classes leave §5d #3867

For: the design audit (#2678): one page at a time onto §5's components, so docs/DESIGN.md §5d only shrinks. Ship is the owner's most frequent decision; its box should be the plainest thing on the page.

Done:

• [ ] A pull's merge box (on the pull page, the browse view and the studio's change page) is drawn from §5's components (src/web/ui.ts helpers such as row, card, badge, btn, and §5 classes) in place of .merge-box .merge-form .merge-headline .merge-sub, which src/web/style/content.ts defines today. The Ship form posts exactly the fields it posts today.
• [ ] Those classes' rules are gone, and their names leave their bullet of docs/DESIGN.md §5d. bun scripts/design-lint.ts passes, and so does --update with a lower or equal baseline.
• [ ] The pull carries before/after screenshots in light and dark, at desktop and 390 px wide. Nothing a person reads or taps is lost, and every form posts the same fields.
• [ ] The page's existing tests pass. A test asserts the old class names are absent from the page's HTML.

Files: src/web/pages/pull.ts, src/web/pages/browse.ts, src/web/studio/change.ts, src/web/style/content.ts, docs/DESIGN.md, the page's test. Risk: another page borrowing one of these classes. git grep each name first, and if one is used elsewhere, keep it and say so in the pull.

Parked by the crew: the check stayed red through 2 automatic tries (pull #3894 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check stayed red through 2 automatic tries (pull #4378 closed).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check stayed red through 2 automatic tries (pull #3894 closed)
2. the check stayed red through 2 automatic tries (pull #4378 closed)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Split into #4398 (the pull page's merge box, markup only), #4399 (the browse view and the studio change page, markup only) and #4400 (the rules and the DESIGN.md names, after both). Three pages, the stylesheet and screenshots in one pull stayed red twice; the sandbox has no browser, so the screenshots are dropped.