Add job-level concurrency (cancel-in-progress) to all four ci-python-app
jobs, matching the ci-python-sam idiom. Per-job group keys include
github.job so the parallel jobs in a single run do not share a group.
Replace sam-deploy starter-template stack-name: $default-branch (which
GitHub substitutes to the literal branch name main) with a
REPLACE-ME-stack-name placeholder, and point cfn-role-arn at the real
shared github-cfn-execution-role.