Feature: Workflow dry-run / simulation mode

Question

## Problem

No way to test workflows without actual execution.

## Proposed Solution

Add dry-run simulation capability:

```
manage(action: "dry-run", workflowId, mockInputs: {...})
```

Simulates execution with mock data, shows path through the graph.

## Why Important

For complex workflows with branching, it's critical to verify all paths before production use.

## Labels

post-alpha