Declare and sync external dependencies with [[deps]] entries

You can now declare external dependency graphs in fabrik.toml using [[deps]] entries, then sync them with fabrik deps sync [name]. Dependency graph sync works for Rust, Swift, Go, and Elixir. In target dependencies, strings still refer to local targets, while inline tables like { cargo = “serde” } point to a named dependency graph. The old fabrik vendor command remains as a hidden alias for compatibility.