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/src/extract/extract-fn.ts

@@ -1,9 +1,7 @@
+import type { ExtractContext, ExtractFn, LlmProvider } from "@uncaged/workflow-runtime";
 import type * as z from "zod/v4";
-
 import { getContentMerklePayload } from "../cas/index.js";
-import type { ExtractContext, LlmProvider } from "../types.js";
 import { llmExtractWithRetry } from "./llm-extract.js";
-import type { ExtractFn } from "./types.js";
 
 /** Builds the user-side extraction prompt (thread + agent output + instruction). */
 export async function buildExtractUserContent(