[D6] `examples/hitl-resume` — pause, approve, resume with checkpoint

[leo-aa88]

Proposed change. Workflow that pauses at a HITL step, persists a checkpoint, resumes on
approval, and shows the checkpoint + audit chain in the trace. Exercises existing
internal/engine/hitl.go / checkpoint.go.

Affected files. new examples/hitl-resume/.

Acceptance criteria. Pause/resume cycle demonstrated deterministically in CI.

Depends on. —