Adds CLAUDE.md documenting that nothing the sandbox does — pushing,
opening a PR, merging one — triggers a workflow run, and that
workflow_dispatch is the only mechanism that does.
Verified on this branch: dispatching ci.yml on a feature branch runs
build, unit tests and lint while skipping the staging and publishing
steps, which are gated on refs/heads/dev. Run 31128906621 passed all
three and skipped both publish steps.
adrianoftyriel