Support standard build envs for CI / Git integrations

CI/CD pipelines often provide variables such as deployment URL, git hash, git branch, etc. We might expose standard exports that can scan them.

Some providers:

Vercel

https://vercel.com/docs/projects/environment-variables/system-environment-variables

VERCEL_URL, VERCEL_BRANCH_URL, VERCEL_GIT_PROVIDER, VERCEL_GIT_REPO_SLUG, VERCEL_GIT_REPO_OWNER, VERCEL_GIT_REPO_ID, VERCEL_GIT_COMMIT_REF, VERCEL_GIT_COMMIT_SHA, VERCEL_GIT_COMMIT_MESSAGE, VERCEL_GIT_COMMIT_AUTHOR_LOGIN, VERCEL_GIT_COMMIT_AUTHOR_NAME, VERCEL_GIT_PREVIOUS_SHA

Netlify

https://docs.netlify.com/configure-builds/environment-variables/

REPOSITORY_URL, BRANCH, HEAD, COMMIT_REF, CACHED_COMMIT_REF, CACHED_COMMIT_REF, PULL_REQUEST, REVIEW_ID, URL, DEPLOY_URL, DEPLOY_PRIME_URL

Github Actions

https://docs.github.com/en/actions/learn-github-actions/variables#default-environment-variables

GITHUB_SHA, GITHUB_ACTOR, GITHUB_API_URL, GITHUB_BASE_REF, GITHUB_HEAD_REF, GITHUB_REF, GITHUB_REF_NAME, GITHUB_REPOSITORY, GITHUB_REPOSITORY_OWNER, GITHUB_REPOSITORY_OWNER_ID, GITHUB_WORKFLOW_SHA

Gitlab CI

https://docs.gitlab.com/ee/ci/variables/predefined_variables.html

CI_COMMIT_AUTHOR, CI_COMMIT_BEFORE_SHA, CI_COMMIT_BRANCH, CI_COMMIT_DESCRIPTION, CI_COMMIT_MESSAGE, CI_COMMIT_REF_NAME, CI_COMMIT_SHA, CI_COMMIT_SHORT_SHA, CI_COMMIT_TAG, CI_ENVIRONMENT_URL, CI_PAGES_URL,

Cloudflare pages

https://developers.cloudflare.com/pages/configuration/build-configuration/#environment-variables

CF_PAGES, CF_PAGES_COMMIT_SHA, CF_PAGES_BRANCH, CF_PAGES_URL

Start with the provider documentation linked in the issue for Vercel, Netlify, GitHub Actions, GitLab CI, and Cloudflare Pages. Compare their listed variables and determine the standard exports and provider-specific mappings needed; the work is done when the supported build environments can be scanned consistently.