Sync dependencies with declarative [[deps]] entries in fabrik.toml

You can now sync external dependencies through declarative [[deps]] entries in fabrik.toml. Run fabrik deps sync [name] to pull a dependency graph, and reference external packages directly in your target dependencies using inline tables like { cargo = "serde" }. This works across Rust, Swift, Go, and Elixir, and the previous fabrik vendor command remains available as a hidden alias for compatibility.