Decouple core Yaak logic from Tauri (#354)

crates/yaak-core/src/lib.rs

@@ -0,0 +1,10 @@
+//! Core abstractions for Yaak that work without Tauri.
+//!
+//! This crate provides foundational types and traits that allow Yaak's
+//! business logic to run in both Tauri (desktop app) and CLI contexts.
+
+mod context;
+mod error;
+
+pub use context::{AppContext, WorkspaceContext};
+pub use error::{Error, Result};