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

packages/workflow-runtime/package.json

@@ -0,0 +1,16 @@
+{
+  "name": "@uncaged/workflow-runtime",
+  "version": "0.2.0",
+  "type": "module",
+  "main": "src/index.ts",
+  "types": "src/index.ts",
+  "scripts": {
+    "test": "bun test"
+  },
+  "peerDependencies": {
+    "zod": "^4.0.0"
+  },
+  "devDependencies": {
+    "zod": "^4.0.0"
+  }
+}