Adds a required-check candidate that selects the tests/e2e test files a PR added or
modified, boots a stage-mirror stack on the runner (migrations, backend, two gateway
processes behind nginx, Postgres, Jaeger, TLS cluster Valkey), and runs those files
three times with retries off. The run job sits behind the e2e-changed GitHub
environment, so a reviewer approves each run before the OIDC token that reads the
provider keys from AWS Secrets Manager exists. Supersedes #34981
