docs: add development guidelines in CLAUDE.md

- Introduce coding standards, testing commands, and tool usage.
- Outline architecture notes, style, and logging conventions.
- Provide guidance on modular design and authentication protocols.

CLAUDE.md

@@ -0,0 +1,24 @@
+# Vault-Hier Development Guidelines
+
+## Build & Test Commands
+- Build & run: `cargo build && cargo run`
+- Run tests: `cargo test` (or `cargo test -- --nocapture` for verbose output)
+- Run single test: `cargo test test_name -- --nocapture`
+- Docker test: `./test_docker.sh` (includes vault initialization)
+- Local test: `./test_local.sh` (sets up local vault)
+- Lint: `cargo clippy -- -D warnings`
+- Format: `cargo fmt --all`
+
+## Code Style Guidelines
+- **Formatting**: Follow rustfmt conventions (run `cargo fmt` before committing)
+- **Imports**: Group by crate (stdlib → external → internal)
+- **Error Handling**: Use `anyhow` with descriptive messages; propagate with `?` or `thiserror` for actionable errors
+- **Naming**: Snake case for functions/variables, CamelCase for types
+- **Async**: Use Tokio for async runtime with structured task management
+- **Logging**: Use `tracing` macros for structured logging (`info!`, `debug!`, `error!`, `warn!`, `trace!`)
+- **Documentation**: Document public APIs with doc comments (`///`)
+
+## Architecture Notes
+- Modular design with separate services (document, vault, API)
+- Hierarchical signing with department validation
+- JWT-based authentication using Vault transit backend