chore: release v4.0.0 - sync 550+ skills and restructure docs

skills/backtesting-frameworks/SKILL.md

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+---
+name: backtesting-frameworks
+description: Build robust backtesting systems for trading strategies with proper handling of look-ahead bias, survivorship bias, and transaction costs. Use when developing trading algorithms, validating strategies, or building backtesting infrastructure.
+---
+
+# Backtesting Frameworks
+
+Build robust, production-grade backtesting systems that avoid common pitfalls and produce reliable strategy performance estimates.
+
+## Use this skill when
+
+- Developing trading strategy backtests
+- Building backtesting infrastructure
+- Validating strategy performance and robustness
+- Avoiding common backtesting biases
+- Implementing walk-forward analysis
+
+## Do not use this skill when
+
+- You need live trading execution or investment advice
+- Historical data quality is unknown or incomplete
+- The task is only a quick performance summary
+
+## Instructions
+
+- Define hypothesis, universe, timeframe, and evaluation criteria.
+- Build point-in-time data pipelines and realistic cost models.
+- Implement event-driven simulation and execution logic.
+- Use train/validation/test splits and walk-forward testing.
+- If detailed examples are required, open `resources/implementation-playbook.md`.
+
+## Safety
+
+- Do not present backtests as guarantees of future performance.
+- Avoid providing financial or investment advice.
+
+## Resources
+
+- `resources/implementation-playbook.md` for detailed patterns and examples.