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/index.ts

@@ -1,48 +1,21 @@
+export { buildDescriptor } from "./bundle/build-descriptor.js";
 export {
-  CROCKFORD_BASE32_ALPHABET,
-  decodeCrockfordBase32Bits,
-  decodeCrockfordToUint64,
-  encodeCrockfordBase32Bits,
-  encodeUint64AsCrockford,
-} from "./base32.js";
-export { buildDescriptor } from "./build-descriptor.js";
-export { validateWorkflowBundle, type WorkflowBundleValidationInput } from "./bundle-validator.js";
-export { type CasStore, createCasStore, createThreadCas } from "./cas.js";
-export { createWorkflow } from "./create-workflow.js";
+  validateWorkflowBundle,
+  type WorkflowBundleValidationInput,
+} from "./bundle/bundle-validator.js";
 export {
-  type ExecuteThreadIo,
-  type ExecuteThreadOptions,
-  executeThread,
-  type PrefilledDiskStep,
-} from "./engine.js";
-export { type ExtractedBundleExports, extractBundleExports } from "./extract-bundle-exports.js";
-export { createExtract, type ExtractFn } from "./extract-fn.js";
-export { getExtractProvider } from "./extract-provider.js";
+  type ExtractedBundleExports,
+  extractBundleExports,
+} from "./bundle/extract-bundle-exports.js";
+export { stringifyWorkflowDescriptor } from "./bundle/generate-descriptor.js";
 export {
-  buildForkPlan,
-  type ForkHistoricalStep,
-  type ForkPlan,
-  type ParsedThreadStartRecord,
-  parseThreadDataJsonl,
-  selectForkHistoricalSteps,
-  tryParseRoleStepRecord,
-  tryParseWorkflowResultRecord,
-} from "./fork-thread.js";
-export { type GcResult, garbageCollectCas } from "./gc.js";
-export { stringifyWorkflowDescriptor } from "./generate-descriptor.js";
-export { hashString, hashWorkflowBundleBytes } from "./hash.js";
-export {
-  type LlmError,
-  llmErrorToCause,
-  llmExtract,
-  llmExtractWithRetry,
-} from "./llm-extract.js";
-export {
-  type CreateLoggerOptions,
-  createLogger,
-  type LogFn,
-  type LoggerSink,
-} from "./logger.js";
+  validateWorkflowDescriptor,
+  type WorkflowDescriptor,
+  type WorkflowRoleDescriptor,
+  type WorkflowRoleSchema,
+} from "./bundle/workflow-descriptor.js";
+export { type CasStore, createCasStore, createThreadCas } from "./cas/cas.js";
+export { hashString, hashWorkflowBundleBytes } from "./cas/hash.js";
 export {
   createContentMerkleNode,
   getContentMerklePayload,
@@ -55,8 +28,36 @@ export {
   type StepMerklePayload,
   serializeMerkleNode,
   type ThreadMerklePayload,
-} from "./merkle.js";
-export { type ReactExtractArgs, reactExtract } from "./react-extract.js";
+} from "./cas/merkle.js";
+export { createWorkflow } from "./engine/create-workflow.js";
+export {
+  type ExecuteThreadIo,
+  type ExecuteThreadOptions,
+  executeThread,
+  type PrefilledDiskStep,
+} from "./engine/engine.js";
+export {
+  buildForkPlan,
+  type ForkHistoricalStep,
+  type ForkPlan,
+  type ParsedThreadStartRecord,
+  parseThreadDataJsonl,
+  selectForkHistoricalSteps,
+  tryParseRoleStepRecord,
+  tryParseWorkflowResultRecord,
+} from "./engine/fork-thread.js";
+export { type GcResult, garbageCollectCas } from "./engine/gc.js";
+export { createThreadPauseGate, type ThreadPauseGate } from "./engine/thread-pause-gate.js";
+export { getWorkerHostScriptPath } from "./engine/worker-entry-path.js";
+export { createExtract, type ExtractFn } from "./extract/extract-fn.js";
+export {
+  type LlmError,
+  llmErrorToCause,
+  llmExtract,
+  llmExtractWithRetry,
+} from "./extract/llm-extract.js";
+export { type ReactExtractArgs, reactExtract } from "./extract/react-extract.js";
+export { getExtractProvider } from "./extract-provider.js";
 export {
   type ExtractProviderConfig,
   getRegisteredWorkflow,
@@ -73,10 +74,7 @@ export {
   type WorkflowRegistryFile,
   workflowRegistryPath,
   writeWorkflowRegistry,
-} from "./registry.js";
-export { err, ok, type Result } from "./result.js";
-export { getDefaultWorkflowStorageRoot, getGlobalCasDir } from "./storage-root.js";
-export { createThreadPauseGate, type ThreadPauseGate } from "./thread-pause-gate.js";
+} from "./registry/registry.js";
 export {
   type AgentBinding,
   type AgentContext,
@@ -101,12 +99,20 @@ export {
   type WorkflowFnOptions,
   type WorkflowResult,
 } from "./types.js";
-export { generateUlid } from "./ulid.js";
-export { getWorkerHostScriptPath } from "./worker-entry-path.js";
-export { type WorkflowAsAgentOptions, workflowAsAgent } from "./workflow-as-agent.js";
 export {
-  validateWorkflowDescriptor,
-  type WorkflowDescriptor,
-  type WorkflowRoleDescriptor,
-  type WorkflowRoleSchema,
-} from "./workflow-descriptor.js";
+  CROCKFORD_BASE32_ALPHABET,
+  decodeCrockfordBase32Bits,
+  decodeCrockfordToUint64,
+  encodeCrockfordBase32Bits,
+  encodeUint64AsCrockford,
+} from "./util/base32.js";
+export {
+  type CreateLoggerOptions,
+  createLogger,
+  type LogFn,
+  type LoggerSink,
+} from "./util/logger.js";
+export { err, ok, type Result } from "./util/result.js";
+export { getDefaultWorkflowStorageRoot, getGlobalCasDir } from "./util/storage-root.js";
+export { generateUlid } from "./util/ulid.js";
+export { type WorkflowAsAgentOptions, workflowAsAgent } from "./workflow-as-agent.js";