A repository's title is named in a new docs/design/areas/repo-head.md, and .repo-title leaves §5d #5775

Done looks like:

• [ ] docs/design/areas/repo-head.md names .repo-title: the repository head's owner / name line, with the owner muted and the slash faint (src/web/style/content.ts). It also says which function draws it.
• [ ] Its line leaves docs/DESIGN.md §5d, and nothing else there changes.
• [ ] bun scripts/design-lint.ts passes, and no stylesheet rule changes.

Files: docs/design/areas/repo-head.md, docs/DESIGN.md. Pattern: #5512. Risks: #4808 retires .owner and .slash. Name only .repo-title, so the two never edit the same line.