Design: a repository's audience mark uses §5's badge, and repo-audience leaves §5d #3852

For: the design audit (#2678): one page at a time onto §5's components, so docs/DESIGN.md §5d only shrinks. A repository's audience mark, which says who can see it (public, private, sealed), is how a reader trusts what they are looking at.

Done:

• [ ] The audience mark in a repository's header (src/web/chrome.ts) is drawn from §5's badge (with its tone) in place of .repo-audience, which src/web/style/content.ts defines today.
• [ ] The rule is gone, and the name leaves its 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 of a public and a private repository's header, in light and dark, at desktop and 390 px wide. The words a reader sees are unchanged.
• [ ] Existing repository-page tests pass. A test asserts repo-audience is absent and the badge says "Private" on a private repository.

Files: src/web/chrome.ts, src/web/style/content.ts, docs/DESIGN.md, a test. Risk: chrome.ts frames every repository page. Touch only the audience mark.

Note for the builder: in src/web/chrome.ts the mark is an <a href="…/settings"> for some readers and a <span> for others, both with a title. Keep the link and the title. Only the styling moves to §5's badge.

Parked by the crew: the check stayed red through 2 automatic tries (pull #3856 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Steward (trevato): pull #3856 was parked by an infrastructure fault, not this work - the egress cap from #3719 stopped every check's package install (02:20-03:12Z). The forge is rolled back and the cap is fixed (c7ea1935). Reopened and re-checked; this strike does not count.

Parked by the crew: the check failed ("the pages > (unnamed)") and the decider saw no point in another try (pull #3856 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 failed ("authorization across origins, mirrors, pins and disconnect > (unnamed)") and the decider saw no point in another try (pull #4423 closed).

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

1. the check failed ("the pages > (unnamed)") and the decider saw no point in another try (pull #3856 closed)
2. the check failed ("authorization across origins, mirrors, pins and disconnect > (unnamed)") and the decider saw no point in another try (pull #4423 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.

Replaced by #4589. Both tries went red on tests the change never touched (migrate-after, origins-reconnect, livetriage, snapshot-mirror, strangers, or an "(unnamed)" hook timeout); those are now planned as #4577–#4582. The fresh issue keeps the diff to the named files and says to run design-lint and lower the baseline in the same pull.