Initial commit: Vault Hierarchical Initializer

This commit adds the full implementation of vault-hier, a Rust utility for:
- Initializing HashiCorp Vault in production mode (non-dev)
- Handling Vault seal/unseal operations with key thresholds
- Using Docker Compose for containerized operation
- Supporting persistent storage via Docker volumes

Key components:
- Rust application for Vault interaction
- Docker and Docker Compose configuration
- Test scripts for local development
- Nix flake for development dependencies

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "vault-hier"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+reqwest = { version = "0.11.18", features = ["json"] }
+tokio = { version = "1.28.0", features = ["full"] }
+serde = { version = "1.0.160", features = ["derive"] }
+serde_json = "1.0.96"
+anyhow = "1.0.70"
+