Add end-to-end testing pipeline

[haseebmalik18]

Summary

ACG side of the e2e pipeline. When a PR is opened here, this workflow hands the PR's head commit to the sandbox repo (codejson-action-e2e), which builds the action from that commit and runs it through all three modes. The workflow waits for the sandbox verdict and its own pass or fail becomes the PR check.

Changes

• e2e.yml dispatches the sandbox workflow with the PR's commit, then polls the run and fails if the sandbox failed
• runs are matched by a correlation id since workflow_dispatch doesn't return a run id
• fork PRs skip the check, they don't get secrets so they can't reach the sandbox

closes #122

[haseebmalik18]

PR blocked until Github App is set up