A Chromium-measured case proves a repository's head page does not scroll sideways at 390x844, and names what overflows when it does #9455

Intent: the second half of #9379, landed only after the wrapping rules are on main.

Done:

• [ ] test/repo-head-fit.test.ts gains a Chromium case like test/apps-fit.test.ts (skipped where no Chromium) that loads the seeded /:owner/:repo at 390x844 and asserts scrollWidth <= innerWidth.
• [ ] When it fails, the message lists the selector and width of each element wider than the window, so the check log says what to fix.
• [ ] If the forge's check measures an overflow, the pull fixes that element in src/web/style/content.ts within its scope (data-repo-head or the page's own classes), not by hiding overflow on body.
• [ ] The pull says in one line what the case printed in the forge's check.