Add CLAUDE.md documentation

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.
+
+## Common Commands
+
+### Building and Testing
+- `make build` - Build the application executable
+- `make test` - Run all tests using Ginkgo test framework
+- `make generate` - Generate mocks and other generated code
+- `make clean` - Clean build artifacts
+
+### Development Tools
+- `make tools` - Install required development tools (ginkgo)
+- `make mockgen-install` - Install mockgen for generating mocks
+
+### Running Single Tests
+- `ginkgo -r -v --focus="specific test pattern"` - Run specific tests matching pattern
+- `go test -v ./...` - Alternative way to run all tests
+
+## Architecture Overview
+
+This is a Go library for managing asynchronous goroutines with enhanced visibility, monitoring, and lifecycle management. The core architecture consists of:
+
+### Core Components
+
+**AsyncRoutineManager** (`async_routine_manager.go`): Central singleton that manages all async routines. Provides registration/deregistration, observer pattern for notifications, and optional monitoring/snapshotting capabilities.
+
+**AsyncRoutine** (`async_routine.go`): Represents a managed goroutine with metadata (name, creation time, status, timebox limits, operation IDs). Supports both regular functions and error group integration.
+
+**AsyncRoutineBuilder** (`async_routine_builder.go`): Fluent builder pattern for creating routines with optional configuration (timebox, custom data, error groups).
+
+### Key Patterns
+
+**Observer Pattern**: `RoutinesObserver` interface allows pluggable monitoring (logging, metrics). Observers get notified on routine start/finish/timeout events.
+
+**Operation ID Tracking** (`opid/`): Each routine gets a unique operation ID for distributed tracing, with support for originator context propagation.
+
+**Prometheus Integration** (`metrics/`): Built-in metrics observer that tracks running routine counts and instances by name using Prometheus gauges.
+
+### Usage Pattern
+```go
+async.NewAsyncRoutine("routine-name", ctx, func() {
+    // your async work
+}).Timebox(5*time.Minute).WithData("key", "value").Run()
+```
+
+The manager can be disabled globally, falling back to standard goroutines. Monitoring includes periodic snapshots and timebox violation detection.