Declarative dependency sync via fabrik.toml

Fabrik now supports declaring external dependencies in fabrik.toml with [[deps]] entries, and syncing them via fabrik deps sync [name]. Dependency graph sync works for Rust, Swift, Go, and Elixir. In target dependencies, strings continue to reference local targets, while inline tables such as { cargo = "serde" } point to a named dependency graph. For Rust, external dependencies are lowered to generated vendor/<deps-name>/<crate> targets, each with its own fabrik.toml declaration.