Depth durable workflows

Depth is BlackLake's durable workflow runtime. Define a workflow with `workflow('name', async (ctx) => { ... })`, run it with `depth run workflow.ts` or `bl run workflow.ts` from the unified package. Every step is a governed evaluation; the `workflow_id`, `run_id`, `step_id`, `attempt` columns on the evaluation row mean the receipt ties back to the runbook. Use Depth when you want governance + durability + cost capture in one runtime. Use the existing workflow-engine adapters above when you already have Temporal/Inngest/etc. running.