feat: Add RL Trading Agent system with 99 tests

Implement Reinforcement Learning trading system inspired by FinRL concepts
(100% original code, NOT copied from FinRL MIT project):

RL ENVIRONMENT:
- TradingEnv: Gymnasium-compatible environment
- State: price history + indicators + portfolio state
- Action: continuous position [-1, 1] (short to long)
- Reward: return - transaction costs - drawdown penalty

RL AGENT:
- RLTradingAgent: Wrapper for Stable Baselines3
- Supports PPO (stable), A2C (fast), SAC (continuous)
- Methods: create_model(), train(), predict(), save(), load(), evaluate()

COSTEER (fills TODO at costeer.py:112):
- RLCosteer: RL-based trading controller
- Risk-limit enforcement (15% drawdown stops trading)
- Position scaling based on risk appetite
- Trade history tracking

TECHNICAL INDICATORS:
- RSI, MACD, Bollinger Bands, CCI, ATR
- prepare_features() helper for easy integration

TESTS (99 total, ALL PASS):
- 26 env tests
- 16 agent tests
- 19 costeer tests
- 18 indicator tests
- 10 integration tests

Documentation:
- Update QWEN.md with RL system architecture

rdagent/components/coder/rl/__init__.py

@@ -1 +1,34 @@
-from rdagent.components.coder.rl.costeer import RLCoSTEER
+"""RL Trading Agent components for Predix.
+
+This package provides:
+- RLCoSTEER: LLM-based code generation for RL training pipelines
+- RLCosteer: RL-based trading controller using trained models
+- TradingEnv: Gym-compatible trading environment
+- RLTradingAgent: Stable Baselines3 wrapper for PPO, A2C, SAC
+- Technical indicators: RSI, MACD, Bollinger Bands, CCI, ATR
+"""
+
+from rdagent.components.coder.rl.agent import RLTradingAgent
+from rdagent.components.coder.rl.costeer import RLCoSTEER, RLCosteer
+from rdagent.components.coder.rl.env import TradingEnv
+from rdagent.components.coder.rl.indicators import (
+    calculate_atr,
+    calculate_bollinger_bands,
+    calculate_cci,
+    calculate_macd,
+    calculate_rsi,
+    prepare_features,
+)
+
+__all__ = [
+    "RLCoSTEER",
+    "RLCosteer",
+    "RLTradingAgent",
+    "TradingEnv",
+    "calculate_atr",
+    "calculate_bollinger_bands",
+    "calculate_cci",
+    "calculate_macd",
+    "calculate_rsi",
+    "prepare_features",
+]