docs: add CLAUDE.md for Claude Code guidance

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: fsouza

CLAUDE.md

@@ -0,0 +1,85 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+fake-gcs-server is a Google Cloud Storage API emulator that can be used as:
+1. A Go library (`github.com/fsouza/fake-gcs-server/fakestorage`) for integration tests
+2. A standalone binary/Docker container for testing GCS-dependent applications in any language
+
+## Common Commands
+
+### Build
+```bash
+go build
+```
+
+### Run Tests
+```bash
+# Run all tests with race detection
+go test -race -vet all -mod readonly ./...
+
+# Run a single test
+go test -race -vet all -mod readonly ./fakestorage -run TestName
+
+# Run tests in a specific package
+go test -race -vet all -mod readonly ./internal/backend
+```
+
+### Lint
+```bash
+golangci-lint run
+staticcheck ./...
+```
+
+### Run the Server
+```bash
+# Default HTTPS on port 4443
+./fake-gcs-server
+
+# HTTP mode
+./fake-gcs-server -scheme http
+
+# Both HTTP and HTTPS
+./fake-gcs-server -scheme both
+
+# With seed data
+./fake-gcs-server -data /path/to/seed/data
+
+# See all flags
+./fake-gcs-server -help
+```
+
+## Architecture
+
+### Package Structure
+
+- **main.go**: CLI entry point, handles server startup with HTTP/HTTPS/both schemes and gRPC multiplexing
+- **fakestorage/**: Public Go library package
+  - `server.go`: Core server implementation, HTTP routing via gorilla/mux
+  - `object.go`, `bucket.go`: GCS resource handlers
+  - `upload.go`: Resumable and multipart upload handling
+- **internal/backend/**: Storage abstraction layer
+  - `storage.go`: `Storage` interface definition
+  - `memory.go`: In-memory backend implementation
+  - `fs.go`: Filesystem-based backend implementation
+- **internal/grpc/**: gRPC server implementation sharing the same backend as HTTP
+- **internal/notification/**: Pub/Sub event notifications
+- **internal/config/**: CLI configuration parsing
+
+### Key Design Patterns
+
+1. **Backend Abstraction**: The `backend.Storage` interface allows swapping between memory and filesystem storage. Both HTTP and gRPC servers share the same backend instance.
+
+2. **HTTP/gRPC Multiplexing**: The main server multiplexes HTTP and gRPC on the same port by checking the Content-Type header for `application/grpc`.
+
+3. **Public Library API**: The `fakestorage` package is designed for use in Go tests - `Server.Client()` returns a pre-configured `*storage.Client` that talks to the fake server.
+
+### API Endpoints
+
+The server implements:
+- JSON API at `/storage/v1/...`
+- XML API for public object access
+- Upload API at `/upload/storage/v1/...`
+- Internal endpoints: `/_internal/healthcheck`, `/_internal/config`, `/_internal/reseed`