Add automotive sales and detailing example

[mintyPT]

Summary

• add a manifest-driven Apex Auto social example set for inventory, detailing, and trade-in posts
• render the output images and contact sheet
• document README usage and Unsplash attributions
• make the quality-gate test timeout explicit for slower browser runs

Closes #28

Validation

• npm run examples:render -- examples/use-cases/automotive-social.manifest.json
• npm run examples:contact-sheet -- examples/use-cases/automotive-social.manifest.json examples/use-cases/automotive-social-contact-sheet.png
• npm test -- automotive-example
• npx vitest run test/quality.test.ts --testTimeout=90000
• npm run check
• npm run build

Note: full npm test was attempted, but local multi-worker browser contention left an existing screenshot-url CLI subprocess stuck in uninterruptible I/O after more than 10 minutes. Earlier full run completed 119/120 tests and failed only the existing quality test timeout, which is addressed here.