docs: sync social-card taglines with repo descriptions

[lesnik512]

Summary

The social-card taglines in brand/build/projects.py::DOCS_REPOS were older one-liners that no longer matched the repo descriptions (httpware's still listed "(retry, bulkhead)"). They now equal the profile row descriptions, which already mirror pyproject and GitHub.

Changes

• Taglines set to the profile descriptions, with a new test (test_every_social_card_tagline_is_its_profile_description) so they cannot drift again. All seven docs-site cards are regenerated.
• The card's aria-label now escapes the tagline. The semvertag description contains &, which produced invalid SVG and failed 15 tests before the fix.
• The aria-label separator is a colon instead of an em dash.
• modern-di-pytest's profile row and docs index entry become "Pytest integration for modern-di that turns DI dependencies into fixtures", matching the pyproject change in modern-di-pytest.

The docs-site repos vendor docs/assets/social-card.png, so each needs its copy refreshed; those copies go out as per-repo PRs.

Checklist

• Lint and format pass (ruff) (no ruff config in this repo)
• Type check passes (ty)
• Tests pass and new behavior is covered (173 passed; mkdocs build --strict passes)
• Build succeeds (uv build) if packaging or build config changed
• Repo metadata stays consistent across the three surfaces (GitHub description, pyproject description, profile blurb) if this touches packaging