fix: Add required metadata to gl-sdk Cargo.toml for crates.io

crates.io requires description and license fields.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cdecker

libs/gl-sdk/Cargo.toml

@@ -2,6 +2,9 @@
 name = "gl-sdk"
 version = "0.2.0"
 edition = "2024"
+description = "High-level SDK for Greenlight with UniFFI language bindings"
+license = "MIT"
+repository = "https://github.com/Blockstream/greenlight"
 
 [lib]
 crate-type = ["cdylib", "staticlib", "rlib"]