docs(examples): add agentic loop demo for Python and Node.js SDKs

Matching examples for all 3 SDKs (Rust already committed):
- Python: sdk/python/examples/agentic_loop_demo.py
- Node.js: sdk/node/examples/agentic_loop_demo.js

Each demo covers 6 scenarios with real LLM integration:
1. Autonomous multi-step coding (create → read → edit → verify)
2. Streaming events (real-time tool/text visibility)
3. Planning mode (task decomposition + goal tracking)
4. Multi-turn conversation (context preservation)
5. Skills-augmented code review (built-in skills)
6. Resilient session (parse retries, timeout, circuit breaker)

Author: ZhiXiao-Lin