feat(workflow): add ThreadReactor generic ReAct loop + migrate extract (Phase 1)

- New src/reactor/ module: createThreadReactor, createLlmFn, types
- Two-stage API: config (llm, systemPrompt, tools, toolHandler) + per-call (thread, input, schema)
- All tool failures are recoverable (returned to LLM as error message)
- Rewrite createExtract to use createThreadReactor
- Delete reactExtract old implementation
- Fix template test imports (START/END from runtime, validateWorkflowDescriptor from engine)

268 tests passing.

Refs #139, relates #140

packages/workflow-template-develop/__tests__/develop-template.test.ts

@@ -1,11 +1,6 @@
 import { describe, expect, test } from "bun:test";
-import {
-  END,
-  type ModeratorContext,
-  type RoleStep,
-  START,
-  validateWorkflowDescriptor,
-} from "@uncaged/workflow-runtime";
+import { END, type ModeratorContext, type RoleStep, START } from "@uncaged/workflow-runtime";
+import { validateWorkflowDescriptor } from "@uncaged/workflow";
 import { buildDevelopDescriptor } from "../src/descriptor.js";
 import { developModerator } from "../src/index.js";
 import type { CommitterMeta, PlannerMeta } from "../src/roles/index.js";