test(engine): add end-to-end Runner.Run pipeline integration tests

[jkleinne]

Summary

Adds the end-to-end Runner.Run() integration test, the last open item on the tech-spec pre-production checklist. The pipeline's orchestration (prerequisite check, per-config lock, job dispatch order, rclone remote expansion, partial-failure accumulation, dry-run, summary assembly) was previously exercised only through leaf-method tests; nothing drove Run() itself.

The new internal/engine/runner_integration_test.go drives the real pipeline against real rsync and rclone's local backend (no mocks, matching the existing executor-test convention), across six scenarios:

• Multi-engine happy path: an rsync job and an rclone job both run and land files.
• Partial failure: a missing source fails, the pipeline continues, and the error is recorded.
• Rclone multi-remote expansion: one job over two remotes yields adjacent per-remote results in declared order.
• Dry-run: nothing is written to the destination.
• --only filtering: the unselected job is skipped and its destination stays untouched.
• Lock contention: a second run against the same config is rejected.