Dependency sync via declarative deps entries in fabrik.toml

The fabrik deps sync [name] command now synchronizes dependency graphs declared as [[deps]] entries in fabrik.toml. You can reference external dependencies inline inside target deps using tables like { cargo = "serde" } for Rust, Swift, Go, or Elixir packages, and Fabrik generates vendor targets under vendor/<deps-name>/. Local target dependencies stay plain strings, while fabrik vendor remains a hidden alias for compatibility.