Add PR metadata validation

[DerekLoadout]

Summary

Adds PR metadata validation for vendor submissions.

Changes

• Validates PR title format.
• Catches unreplaced PR template placeholders.
• Requires completed shop, website, and community rows.
• Requires vendor confirmations for add/update PRs only.
• Expands workflow path coverage for validation tooling.

Testing

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

[DerekLoadout]

Closing this in favor of the clean replacement PR #45. This branch included unrelated vendor-listing history, so #45 contains only the validation/docs/tooling changes.