contrib/jenkins.sh: add support for "Jenkins: skip-card-tests" marker
Add a have_jenkins_marker() helper that checks the commit message for a given value on the "Jenkins:" footer line, for instance:
Jenkins: skip-card-tests
When this marker is present, the real-card integration tests (pySim-prog_test and pySim-shell_test) are skipped, allowing commits that don't affect card I/O to bypass long card-dependent test runs.
