Adds `make migration-lint` (stapel_tools.migration_lint . --strict) and a
CI step next to Lint/Import check. Zero migrations findings today. The CI
step ships commented-out with a TODO: no module repo's CI installs
stapel-tools yet (not on PyPI under the new org, no git-install mechanism
wired anywhere). Run `make migration-lint` locally / at release-cut time
until CI can fetch stapel-tools.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>