mvb fails on fork PRs: branch doesn't exist in upstream repo

[AaronFeledy]

Problem

When running mvb in a GitHub Actions workflow triggered by a PR from a fork, the build fails because mvb tries to checkout the PR's branch name in the upstream repo.

Error

@lando/mvb running command 'git' [ 'checkout', '53-ruby-4.0', '--force' ] from '/tmp/...'
@lando/mvb stderr error: pathspec '53-ruby-4.0' did not match any file(s) known to git

Cause

mvb clones the upstream repo to a temp directory, then tries to checkout the branch by name. For fork PRs, the branch (e.g., 53-ruby-4.0) only exists in the contributor's fork, not in the upstream repo.

Reproduction

1. Contributor opens PR from their fork (e.g., contributor/repo → lando/repo)
2. GHA workflow runs npx mvb docs
3. mvb clones lando/repo and tries git checkout <fork-branch-name>
4. Fails because that branch doesn't exist in lando/repo

Example

• PR: Support Ruby 4.0 and mark 3.1 as legacy ruby#54
• Failed run: https://github.com/lando/ruby/actions/runs/21226013103

Suggested Fix

For fork PRs, mvb should either:

1. Clone the fork's repo URL instead of upstream
2. Use the PR merge ref (refs/pull/N/merge) instead of branch name
3. Detect fork PRs and skip the dev branch checkout