feat: introduce Rig agent runtime migration

crates/galaxy_agent_core/src/lib.rs

@@ -0,0 +1,11 @@
+//! Provider- and UI-independent contracts for Galaxy agent runtimes.
+//!
+//! This crate is the stable boundary between Galaxy application services and
+//! concrete runtimes such as Rig-backed providers or ACP agents. It must not
+//! depend on GalaxyUI, provider SDKs, persistence, or Warp wire protocols.
+
+mod runtime;
+mod types;
+
+pub use runtime::*;
+pub use types::*;