Add minimal Hello World smart program (Gear)

[A-mont]

What

• Introduces a minimal Gear smart program under contracts/hello_world that replies with a friendly greeting.

Why

• Provides a simple, self-contained example for internal training and a quick starting point to understand message handling and replies in Gear smart programs.

How

• A no_std cdylib WASM program using gstd that:
  • Stores an optional greeting on init (default: "Hello").
  • On handle, reads a String name (default: "World") and replies with ", !".
  • On state, returns the current greeting.
• Program is standalone and does not modify any existing workspace files. It can be built independently.

How to test

1. Build

• Ensure you have the wasm target: rustup target add wasm32-unknown-unknown
• Build the program: cargo build --release --target wasm32-unknown-unknown --manifest-path contracts/hello_world/Cargo.toml
• The compiled artifact will be at: contracts/hello_world/target/wasm32-unknown-unknown/release/hello_world.wasm

2. Deploy (example using any Gear-compatible tool)

• Upload the WASM to a local node or testnet via your preferred UI/tooling.
• Init payload: optional String (SCALE-encoded). If omitted, the greeting defaults to "Hello".

3. Interact

• Send a handle message with a String payload (SCALE-encoded), e.g. "Alice".
• Expected reply: "Hello, Alice!" (or using your custom greeting if set during init).
• Query the state function to retrieve the current greeting string.

Notes

• If your workspace requires adding crates explicitly, you can include this crate into the workspace members in the top-level Cargo.toml (not included here to keep changes minimal).
• TODO: Add metadata generation (gmeta) and simple integration tests if needed.