feat(langchain): add LangChain export adapter & runner with multi-provider support

README.md

@@ -359,6 +359,7 @@ Adapters are used by both `export` and `run`. Available adapters:
 | `opencode` | OpenCode instructions + config |
 | `openclaw` | OpenClaw format |
 | `nanobot` | Nanobot format |
+| `langchain` | LangChain agent Python code |
 
 ```bash
 # Export to system prompt

src/adapters/index.ts

@@ -6,3 +6,4 @@ export { exportToOpenClawString, exportToOpenClaw } from './openclaw.js';
 export { exportToNanobotString, exportToNanobot } from './nanobot.js';
 export { exportToCopilotString, exportToCopilot } from './copilot.js';
 export { exportToOpenCodeString, exportToOpenCode } from './opencode.js';
+export { exportToLangChain } from './langchain.js';