Add CLAUDE.md for AI coding assistants

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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
+
+CloudServer is Scality's open-source, AWS S3-compatible object storage server.
+It provides a single S3 API interface to access multiple storage backends
+(Scality RING, file, memory, AWS S3, Azure, GCP, etc.). Part of the
+S3C/Federation and Zenko stacks.
+
+- **Node.js >= 22** required
+- **Package Manager**: Yarn with `--frozen-lockfile`
+
+## Common Commands
+
+### Development
+
+```bash
+yarn install --frozen-lockfile     # Install dependencies
+yarn start                         # Start S3 + metadata + data servers
+yarn dev                           # Development mode with auto-restart (nodemon)
+yarn mem_backend                   # Start with in-memory backend (S3BACKEND=mem)
+yarn start_mongo                   # Start with MongoDB metadata backend
+```
+
+### Testing
+
+```bash
+# Unit tests
+yarn test                          # Run unit tests (S3BACKEND=mem)
+yarn cover test                    # Run unit tests with coverage
+
+# Functional tests (require running server)
+yarn ft_awssdk                     # AWS SDK functional tests
+yarn ft_awssdk_buckets             # Bucket-specific AWS SDK tests
+yarn ft_awssdk_versioning          # Versioning tests
+yarn ft_s3cmd                      # S3cmd CLI tests
+yarn ft_healthchecks               # Health check tests
+yarn ft_test                       # Run all functional tests
+
+# Multi-backend tests
+yarn multiple_backend_test         # Multi-backend unit tests
+yarn ft_awssdk_external_backends   # External backend functional tests
+```
+
+### Linting
+
+```bash
+yarn lint                          # ESLint on all JS files
+yarn lint_md                       # Markdown linting
+```
+
+## Architecture
+
+### Entry Points
+
+- `index.js` → Main S3 API server (port 8000)
+- `mdserver.js` → Metadata server (port 9990)
+- `dataserver.js` → Data storage server (port 9991)
+- `pfsserver.js` → Passthrough filesystem server (port 9992)
+- `managementAgent.js` → WebSocket management agent (port 8010)
+
+### Core Components
+
+```
+lib/
+├── server.js          # S3Server class - main HTTP server with cluster support
+├── Config.js          # Configuration management (env vars, config.json)
+├── api/               # S3 API implementations (80+ operations)
+│   ├── api.js         # Main request router and auth handler
+│   └── apiUtils/      # Shared utilities (auth, bucket ops, object ops, quotas)
+├── auth/              # Authentication (vault.js for IAM, in_memory/ for local)
+├── data/              # Data backend abstraction (file, memory, multiple)
+├── metadata/          # Metadata backend abstraction
+├── routes/            # Special routes (Backbeat, metadata service, Veeam)
+├── kms/               # Key management (file, memory, AWS, KMIP)
+├── management/        # Management API and configuration
+└── utilities/         # Logging, health checks, XML parsing
+```
+
+### Backend Configuration
+
+Set via environment variables. Each backend type abstracts multiple
+implementations:
+
+### Data Backends (`S3DATA`)
+
+| Backend | Port | Description |
+|---------|------|-------------|
+| `file` | 9991 | Local filesystem via `dataserver.js` |
+| `multiple` | - | Multi-backend gateway (AWS S3, Azure, GCP, Sproxyd) |
+| `mem` | - | In-memory (testing only) |
+
+`scality` is an alias for `multiple`. With `multiple`, objects route to
+backends defined in `locationConfig.json` based on location constraints.
+
+### Metadata Backends (`S3METADATA`)
+
+| Backend | Port | Description |
+|---------|------|-------------|
+| `file` (default) | 9990 | Local LevelDB via `mdserver.js` |
+| `scality` | 9000 | External bucketd service (production Scality RING) |
+| `mongodb` | 27017+ | MongoDB replica set |
+| `mem` | - | In-memory (testing only) |
+
+**file vs scality**: The `file` backend runs a self-contained metadata server
+(`mdserver.js`) for development. The `scality` backend connects to external
+**bucketd** which uses Raft-based distributed metadata (repd) for production
+HA deployments.
+
+### Auth Backends (`S3VAULT`)
+
+| Backend | Port | Description |
+|---------|------|-------------|
+| `mem` (default) | - | In-memory accounts from `conf/authdata.json` |
+| `vault` | 8500 | External Vault IAM service (vaultd) |
+
+### KMS Backends (`S3KMS`)
+
+| Backend | Description |
+|---------|-------------|
+| `file` (default) | Local file-based key storage |
+| `mem` | In-memory (testing only) |
+| `kmip` | External KMIP server |
+| `aws` | AWS KMS |
+| `scality` | Scality KMS |
+
+### Path Configuration
+
+- `S3DATAPATH`: Data storage directory (default: `./localData`)
+- `S3METADATAPATH`: Metadata storage directory (default: `./localMetadata`)
+
+### Key Files
+
+- `config.json` - Default configuration
+- `constants.js` - Global constants (splitters, limits, service accounts)
+- `locationConfig.json` - Location constraints for multi-backend
+
+## Testing Conventions
+
+- **Framework**: Mocha with Sinon for mocking
+- **Timeout**: 40000ms global
+- **Coverage**: NYC (Istanbul), 80% patch target
+- **Reporters**: mocha-multi-reporters (spec + junit XML)
+
+Run a single test file:
+
+```bash
+S3BACKEND=mem yarn mocha tests/unit/path/to/test.js --exit
+```
+
+Run tests matching a pattern:
+
+```bash
+S3BACKEND=mem yarn mocha --grep "pattern" tests/unit --recursive --exit
+```
+
+## Code Style
+
+- ESLint with `@scality/eslint-config-scality`
+- `mocha/no-exclusive-tests: error` - Never commit `.only()` tests
+- Many formatting rules disabled for legacy compatibility
+- ECMAScript 2021, CommonJS modules (`sourceType: "script"`)