Add PR metadata validation

[DerekLoadout]

Summary

Adds PR metadata validation for vendor submissions.

Changes

• Validates vendor PR title format.
• Catches unreplaced PR template placeholders.
• Requires completed shop, website, and community rows for vendor listing PRs.
• Requires vendor confirmations for add/update PRs only.
• Skips vendor metadata checks for maintenance PRs that do not touch vendors/ or logos/.
• Expands workflow path coverage for validation tooling.
• Documents accepted vendor PR title prefixes.

Testing

• npm run validate
• npm run validate:pr
• node --check scripts/validate-pr.mjs
• Sample Add vendor / Deactivate vendor payloads