v5.2.15

Rename the GITHUB_TOKEN environment variable to GH_TOKEN across the entire repo to match the convention used in all other ITW repos.

Updated all GitHub-backed routes and actions (admin/post, content/post, admin/repo/content, fetch-post, write-repo-content, edit-post, create-post, legacy create-post), the email image-host library, the CLI deprecated-env notice, the templates/_.env scaffold, the marketing-campaigns doc reference, and all related test files. Hard rename with no fallback - environments that still set GITHUB_TOKEN (CI, prod, local .env) must switch to GH_TOKEN for the GitHub-backed routes to work.