A mirrored repo's page names its upstream, its licence and its original authors #10531

Part of #10447.

Done when:

• [x] The repo page of a read-only mirror shows one quiet line: the upstream link, the licence (read from LICENSE/COPYING at the tip, else "no licence file"), and the top original authors from its history.
• [x] A fork of a mirror shows the same credit through its lineage.
• [ ] A test renders the page for a mirror and a fork of it and finds all three; the output is escaped. bun scripts/design-lint.ts passes; before/after screenshot in the pull.

Reviewed pull #10847 at its head: ticked The repo page of a read-only mirror shows one quiet line: the upstream link, the licence (read from LICENSE/COPYING at the tip, else "no licence file"), and the top original authors from its history.; A fork of a mirror shows the same credit through its lineage.. left open: A test renders the page for a mirror and a fork of it and finds all three; the output is escaped. bun scripts/design-lint.ts passes; before/after screenshot in the pull. — Tests and design-lint are in place, but the pull has no before/after screenshot..

Left open after pull #10847 landed: 1 unticked item remains (A test renders the page for a mirror and a fork of it and finds all three; the output is escaped. bun scripts/design-lint.ts passes; before/after screenshot in the pull.). The planner files each as its own issue and closes this one; a person may close it, tick what is done, or say what is left. Until then the crew does not pick it again.

Closed by the planner: the test box is met by test/mirror-credit.test.ts (a mirror and a fork of it, credit escaped). The design-lint and screenshot box is not wanted as its own run on a shipped page.

Replaced by #10868: the test box is met on main by test/mirror-credit.test.ts (a mirror and a fork of it, escaped); the design-lint and looks box moves there.