ci: integration tests for crontab backend

[awbx]

Split off from #10. Real-backend integration tests gated by the integration build tag, like the kubernetes reference shipped in #10's PR.

What ships

• go/internal/backends/crontab/integration_test.go with //go:build integration and tests covering Create → List → Update → Delete and at least one drift/divergence scenario.
• A new job in .github/workflows/integration.yml that brings up a scratch user's crontab on the runner and runs go test -tags integration -run Integration ./internal/backends/crontab/....
• Documentation pointer at the top of the test file explaining how to run locally.

Reference

See go/internal/backends/kubernetes/integration_test.go and the matching kubernetes job in .github/workflows/integration.yml shipped in #10. Same shape for every backend: per-test namespace/scope for isolation, t.Cleanup tears down, three scenarios minimum (CRUD, drift/divergence, unmanaged-entries-untouched).

Acceptance criteria

• Tests cover the three scenarios from the kubernetes reference
• Tests run cleanly on a fresh GitHub-hosted runner
• Job adds <2 minutes of CI wall-clock to a typical PR
• Per-test isolation: rerunning the same test multiple times against the same backend instance succeeds
• The integration test file is excluded from default go test ./... (verify by running without the build tag)