Add Gitcoin Grants Round 18 campaign entry

[2js56]

/claim #301

Summary

• Add the Gitcoin Grants Round 18 campaign entry.
• Add GBOYEE to authors.json so the submitted author resolves during validation.
• Generate the required campaign banner at /content-images/campaigns/gitcoin-grants-round-18/banner.png.
• Use official GG18 dates and public Gitcoin references instead of the issue's draft March 2024 wording.

Validation

• node -e "JSON.parse(require('fs').readFileSync('src/data/authors.json','utf8')); console.log('authors ok')"
• npx tsx scripts/validate-content.ts src/content/campaigns/gitcoin-grants-round-18.md
• git diff --check

Fixes #301.

[vercel]

Someone is attempting to deploy a commit to the Allo Capital Team on Vercel.

A member of the Team first needs to authorize it.