v0.12.25: @newsroom.newsletter: the end of every issue names the newsroom, and the LinkedIn box names the cover alone

The first release from the newsletter session, which writes the SGit Newsroom newsletter from issue 6 onwards.

Every issue page ends with a paragraph that is selected and copied into LinkedIn with the body, so it is also how every LinkedIn post of an issue ends. Its links already pointed here, but its words still said "sgit.ai" and "subscribe at sgit.ai/subscribe". It now reads: "Every article it links to is on newsroom.sgit.ai, with its sources and its data. To get the next issue by email, subscribe at newsroom.sgit.ai/subscribe", with both links absolute, because the paragraph is pasted.

Under it, the "Posting this issue on LinkedIn?" box listed every image in the issue, with captions, as an upload order, because LinkedIn used to drop images from a paste. Copy and paste of images works now, so the box keeps the cover and one line: upload the cover, paste the title, copy the body, and its images copy with it.

Both changes are made on import, as item 15 of sync_newsroom.py, because the issue pages are still rendered by sgit.ai's generator, which this repository borrows and does not edit. The import stops if an issue page has the paragraph or the box other than exactly once, so a change on sgit.ai's side is noticed rather than missed. A new test, admin/build/test_issues.mjs, checks every issue page and its markdown twin. When the generator moves here, the change goes into issue_body() and linkedin_kit() and the import rule is retired.