chore: untrack .claude, guard against AI co-authors, ruled blog grid

[CodeWithEugene]

Repo hygiene

• Stop tracking the local .claude/ directory (launch config, proven-config files) and ignore it. The files stay on each machine.
• New commitlint rule no-ai-coauthor: any commit with a Co-authored-by trailer naming Claude/Anthropic fails the Conventional Commits check. PR commits without the trailer also keep GitHub's squash merge from adding one, which is how Claude ended up credited (PR feat(ui): rebuild landing page on a Stripe-style grid #53's first squash commit, since force-pushed away).

UI

• Blog grid (/blog and Keep Reading): square cells in a ruled band that runs out to the frame's side rules, separated by shared hairlines instead of separate rounded cards. 3/2/1 columns with the right rule dropped on each row's last cell.
• Footer on mobile: "In partnership with" and the Salamander logo get their own centered row; desktop keeps it under Contact Us.

Test plan

• tsc, lint, 471 tests, build (clean worktree)
• commitlint rejects a Claude co-author trailer, accepts a human one
• Blog grid checked at 1440px (edges flush with the frame rules) and 375px; footer checked at both widths