v0.13.7: @newsroom.writer: a faster build

A release took about 12.5 minutes to build locally and about 10.5 minutes in CI. Measured step by step, half the local time was the import through sgit.ai's generator, and the other half was the 18 tests run one after another.

• Tests in parallel. A new admin/build/run_tests.sh runs every admin/build/test_*.mjs side by side. Each test serves on a port the system picks and drives its own browser. The two tests that measure timing, test_themes and test_reading, run alone afterwards, because under load test_themes read a page mid-switch. A test that prints SKIP fails. Locally the tests drop from about 355 s to about 170 s. CI now runs the same script four at a time, and also runs test_issues, which it had not listed.
• Fewer generator builds. admin/build/publish.py seeds the clone with the cards and banners from the last release, so the first build of sgit.ai's generator passes when no article is new. The second and third builds now run only when a pass adds a card or a banner. Measured: 357 s down to 192 s.
• A Chromium cache in CI. Playwright's Chromium is cached between runs; the system libraries still come from apt.
• Only new tags pushed. The release tag job pushes only the tags it made, not all of them again, which had grown to over a minute a run.
• The roadmap's summaries are re-stamped. These are the summaries of the proposed RiskMandate roadmap, after v0.13.6's wording.