Update campaign workflows and add Codex skill

[kenlim-mops]

Summary

• add X organic and paid presets while preserving the governed canonical source values
• preserve the POC email sign-in path and include database migrations in Vercel functions
• add the scalable campaign picker and audited campaign-to-initiative reassignment
• add the repository-scoped $utm-builder-v2 Codex skill and safe MCP operating workflow
• document Codex skill discovery, installation, secure MCP setup, verification, troubleshooting, rollout, administration, and architecture boundaries

Validation

• 21 test files / 139 tests passed
• TypeScript check passed
• production build passed, including its lint/type phase
• Codex skill validator passed
• openai.yaml parsed successfully
• all local Markdown links in the changed documentation resolve
• git diff --check passed

Lint command note

npm run lint currently opens Next.js's interactive first-time ESLint configuration because this repository does not contain an ESLint configuration. That prompt was canceled rather than changing lint policy as part of this work. The production build completed its integrated lint/type phase successfully.

Merge requirement

The repository requires at least one approving review from a reviewer with write access before merging to main.