refactor: organize workflow/src into 6 module folders

Move 34 flat modules into cas/, registry/, bundle/, extract/, engine/, util/.
Move gc.ts to engine/ (was in cas/) to avoid cas→engine reverse dependency.
Dependency direction: util ← cas ← extract ← engine, util ← registry ← bundle.
No logic changes — only file locations and import paths.

Refs #102

packages/workflow/src/extract/extract-fn.ts

@@ -0,0 +1,63 @@
+import type * as z from "zod/v4";
+import { getContentMerklePayload } from "../cas/merkle.js";
+import type { ExtractContext, LlmProvider } from "../types.js";
+import { llmExtractWithRetry } from "./llm-extract.js";
+
+export type ExtractFn = <T extends Record<string, unknown>>(
+  schema: z.ZodType<T>,
+  prompt: string,
+  ctx: ExtractContext,
+) => Promise<T>;
+
+/** Builds the user-side extraction prompt (thread + agent output + instruction). */
+export async function buildExtractUserContent(
+  ctx: ExtractContext,
+  prompt: string,
+): Promise<string> {
+  const lines: string[] = [];
+  lines.push(`## Role: ${ctx.currentRole.name}`);
+  lines.push(ctx.currentRole.systemPrompt);
+  lines.push("");
+  lines.push("## Task");
+  lines.push(ctx.start.content);
+  lines.push("");
+  if (ctx.steps.length > 0) {
+    lines.push("## Thread History");
+    for (const step of ctx.steps) {
+      const body = await getContentMerklePayload(ctx.cas, step.contentHash);
+      if (body === null) {
+        throw new Error(`extract: missing CAS blob for step ${step.role}: ${step.contentHash}`);
+      }
+      lines.push(`### ${step.role}`);
+      lines.push(body);
+      lines.push(`Meta: ${JSON.stringify(step.meta)}`);
+      lines.push("");
+    }
+  }
+  lines.push("## Agent Output");
+  lines.push(ctx.agentContent);
+  lines.push("");
+  lines.push("## Extraction Instruction");
+  lines.push(prompt);
+
+  return lines.join("\n");
+}
+
+/**
+ * Create an ExtractFn backed by an LLM provider.
+ * Builds prompt text from {@link ExtractContext} plus `prompt` and calls structured extraction.
+ */
+export function createExtract(provider: LlmProvider): ExtractFn {
+  return async <T extends Record<string, unknown>>(
+    schema: z.ZodType<T>,
+    prompt: string,
+    ctx: ExtractContext,
+  ): Promise<T> => {
+    const text = await buildExtractUserContent(ctx, prompt);
+    const result = await llmExtractWithRetry({ text, schema, provider });
+    if (!result.ok) {
+      throw new Error(`extract failed: ${JSON.stringify(result.error)}`);
+    }
+    return result.value;
+  };
+}