docs: Add CLAUDE.md in repo (#202)

Signed-off-by: Xuanwo <github@xuanwo.io>

Author: Xuanwo

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+BackON is a Rust retry library that provides a fluent API for implementing retry logic with various backoff strategies. The library supports both synchronous and asynchronous operations, with cross-platform support including WASM and no_std environments.
+
+## Core Architecture
+
+- **Backoff Strategies** (`backon/src/backoff/`): Different retry timing strategies
+  - `ExponentialBackoff`: Exponential backoff with optional jitter
+  - `ConstantBackoff`: Fixed delay between retries
+  - `FibonacciBackoff`: Fibonacci sequence-based delays
+  - All implement the `Backoff` trait defined in `api.rs`
+
+- **Sleep Implementations** (`backon/src/sleep.rs`, `backon/src/blocking_sleep.rs`): Platform-specific sleep implementations
+  - Async sleepers: `TokioSleeper`, `GlooTimersSleep`, `FutureTimerSleep`, `EmbassySleeper`
+  - Blocking sleepers: `StdSleeper`
+  - All sleepers implement `Sleeper` or `BlockingSleeper` traits
+
+- **Retry Logic** (`backon/src/retry.rs`, `backon/src/blocking_retry.rs`): Core retry functionality
+  - `Retryable` trait for async functions
+  - `BlockingRetryable` trait for sync functions
+  - `RetryableWithContext` for functions that need mutable context
+
+- **Features**: Conditional compilation based on target and enabled features
+  - Default features enable common sleepers for different platforms
+  - WASM-specific implementations using `gloo-timers`
+  - Embassy support for embedded environments
+
+## Development Commands
+
+```bash
+# Check code for errors
+cargo check
+
+# Build the project
+cargo build
+
+# Run linting (clippy)
+cargo clippy
+
+# Run tests
+cargo test
+
+# Run benchmarks
+cargo bench
+
+# Format code
+cargo fmt
+```
+
+## Testing
+
+- Unit tests are in each module alongside the implementation
+- Integration tests demonstrate real-world usage patterns
+- WASM tests use `wasm-bindgen-test`
+- Platform-specific tests are gated by target architecture
+
+## Key Patterns
+
+- Builder pattern for backoff strategies (e.g., `ExponentialBuilder::default()`)
+- Trait-based design for extensibility (`Backoff`, `Sleeper`, `Retryable`)
+- Feature-gated implementations for different platforms
+- Zero-cost abstractions with compile-time feature selection
+- No-std compatibility with optional std features