CI: Fork PRs always fail Vercel deployment check, blocking external contributions

[andrewm4894]

Problem

External contributors who open PRs from forks will always see a failing Vercel – posthog-nextjs-config CI check. Vercel deliberately withholds environment variables (like ENVIRONMENT_ID) from fork builds as a security measure to prevent secret exfiltration.

This creates a poor contributor experience:

1. Contributor confusion — They see a red X and may think their code is broken, when it's actually a CI infrastructure limitation they can't fix
2. Blocked merging — If the Vercel check is required in branch protection, the PR can't be merged even though all actual code quality checks pass
3. Extra maintainer overhead — We have to manually re-push the branch to the main repo and open a new PR to get CI to pass

The call-flags-project / add-to-project-board check also fails on fork PRs due to GitHub permissions, adding to the noise.

Example

PR #2510 from @Tehnix had all code checks passing (tests, lint, TypeScript, Playwright, etc.) but couldn't be merged due to the Vercel check failing with:

Build error occurred
[Error: Environment ID not provided. If you are using turbo, make sure to add env variables to your turbo config]

We had to push the branch to the main repo and open #2975 to work around it.

Possible solutions

A few approaches other repos use — not mutually exclusive:

1. Make the Vercel check non-required in branch protection rules. It still runs and shows a warning, but doesn't block merging.
2. Use a "safe to test" label workflow — a maintainer reviews the fork PR, adds a label, and that triggers a re-run from the main repo context with secrets available.
3. Document it — add a note in CONTRIBUTING.md explaining that the Vercel preview check will fail on fork PRs and that's expected.
4. Split required vs informational checks — only mark code quality checks (tests, lint, build, TypeScript) as required. Keep Vercel and project board checks as optional/informational.

Option 4 is probably the least friction while keeping security intact. The Vercel preview is useful for review but shouldn't gate merging.

[andrewm4894]

not 100% sure if this is valid or not but made an issue for if/when comes up again