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
xiaoju
a7171f05f6