refactor(workflow): split @uncaged/workflow-runtime from engine (Phase 1)

Create packages/workflow-runtime with the minimal runtime subset:
- Types (WorkflowFn, RoleOutput, AgentBinding, etc.)
- createWorkflow (pure orchestration, zero I/O)
- validateWorkflowDescriptor
- Result/ok/err, START/END constants

Zero external dependencies (zod as peer only).
Zero node:fs/node:path imports.

Engine (@uncaged/workflow) now depends on workflow-runtime and
provides CAS/merkle/extract implementations via injection.

Refs #121, relates #122

tsconfig.json

@@ -17,6 +17,7 @@
     "types": ["bun-types"]
   },
   "references": [
+    { "path": "packages/workflow-runtime" },
     { "path": "packages/workflow" },
     { "path": "packages/workflow-agent-llm" },
     { "path": "packages/workflow-agent-cursor" },