docs: add CLAUDE.md for Claude Code guidance

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
+
+A Kubernetes monitoring tool that checks for CronJobs not running according to schedule and sends Slack notifications. It runs as a container inside a K8s cluster, polling all CronJobs every 60 seconds.
+
+## Common Commands
+
+```bash
+# Run tests
+go test ./...
+
+# Run tests with race detection and coverage
+go test -race -coverprofile=coverage.txt ./...
+
+# Build the binary
+CGO_ENABLED=0 go build -o release/cron-checker -ldflags '-w -s'
+
+# Check for vulnerabilities
+govulncheck ./...
+
+# Format and vet
+go fmt ./...
+go vet ./...
+
+# Check for unused modules
+go mod tidy
+```
+
+## Architecture
+
+This is a single-file Go application (`main.go`) with comprehensive tests (`main_test.go`).
+
+**Core Components:**
+- `doCheck()` - Main loop that polls K8s CronJobs, parses cron schedules, and sends Slack alerts for overdue jobs
+- `Client` / `ClientProvider` interfaces - Abstractions over K8s client for testability
+- `ConfigProvider` / `InClusterProvider` - Handles K8s in-cluster authentication
+
+**Key Dependencies:**
+- `k8s.io/client-go` - Kubernetes API client
+- `github.com/robfig/cron` - Cron schedule parsing
+- `github.com/multiplay/go-slack` - Slack webhook integration
+- `github.com/alecthomas/kingpin/v2` - CLI flag parsing
+
+**Configuration:**
+- `SLACK_URL` env var or `--slack-url` flag (required) - Slack webhook URL for notifications
+
+## Building Docker Image
+
+The Dockerfile performs a multi-stage build that runs fmt, vet, and tests before building:
+
+```bash
+docker build -t cron-checker .
+```
+
+To extract coverage report:
+```bash
+docker build --target export -o . .
+```