feat(appkit): add AgentPlugin for LangChain/LangGraph agents

[hubertzub-db]

Adds a new first-class plugin for building AI agents with:

• AgentInterface contract decoupled from LangGraph internals
• StandardAgent wrapping LangGraph createReactAgent with full Responses API SSE streaming (sequence numbers, output indices)
• Invoke handler supporting string/array input, chat history with function_call normalization, and /invocations + /responses routes
• Bring-your-own-agent via config.agentInstance escape hatch
• MLflow tracing integration via OTel env vars and LangChain instrumentation
• MCP tool loading with graceful fallback
• injectAppRoutes hook on base Plugin class for root-level routes
• 33 tests (unit + integration) using deterministic StubAgent
• Cross-repo test script for validating against agent-langchain-ts template