Add dependency sync entries

Projects can now declare dependency graphs declaratively inside fabrik.toml using [[deps]] entries and sync them with fabrik deps sync <name>. Dependency graph sync works for Rust, Swift, Go, and Elixir projects. In target dependency lists, plain strings still refer to local targets, while inline tables like { cargo = "serde" } point at the named external dependency graph. Rust external dependencies are lowered into generated vendor/<deps-name>/<crate> targets, with generated declarations written under vendor/<deps-name>/fabrik.toml. The fabrik vendor command remains available as a hidden alias for compatibility.