docs: add CLAUDE.md with project overview and build instructions

CLAUDE.md

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+# fesen-httpclient
+
+HTTP client library for OpenSearch/Elasticsearch. Provides a unified Java API
+that works with OpenSearch 1.x/2.x/3.x and Elasticsearch 7.x/8.x over HTTP.
+
+## Build & Test
+
+```bash
+mvn install                    # Build and run tests
+mvn install -DskipTests        # Build without tests
+mvn test                       # Run tests only (requires Docker)
+```
+
+Tests use Testcontainers and require Docker to be running.
+
+## Project Structure
+
+- `src/main/java/.../client/` - Core client classes
+  - `HttpClient.java` - Main client implementation
+  - `HttpAbstractClient.java` - Base class with shared logic
+  - `HttpAdminClient.java` / `HttpIndicesAdminClient.java` - Admin APIs
+  - `EngineInfo.java` - Backend engine detection (OpenSearch vs Elasticsearch)
+  - `action/` - HTTP action implementations for each API endpoint
+  - `curl/` - Low-level HTTP request handling
+  - `node/` - Node discovery and management
+- `src/test/java/` - Integration tests per backend version
+
+## Key Details
+
+- Java 17+ required (compiler target)
+- Supports multiple backends: OpenSearch 1/2/3, Elasticsearch 7/8
+- Uses maven-release-plugin for versioning
+- Run `mvn formatter:format && mvn license:format` before committing