Configure a Spacelift push policy to ignore changes to non-IaC files

Steps

1. Create a new `Push` type policy in Spacelift; push policies receive a git push payload including changed file paths as `input.push.affected_files`.
2. Write a Rego rule that checks whether any changed file matches a path prefix for IaC directories: `track { input.push.affected_files[_] == file; startswith(file, "infra/") }`.
3. Add an `ignore` rule that fires when no IaC file is changed: `ignore { count([f | f := input.push.affected_files[_]; startswith(f, "infra/")]) == 0 }`.
4. Attach the push policy to the target stack and push a commit that changes only a markdown file; confirm the run is ignored in the Spacelift UI.
5. Test the policy logic in the Spacelift policy simulator before attaching it to production stacks.
6. Combine with a `Trigger` policy to automatically start downstream stacks when a shared module changes.