refactor(v2): extract bterminal-core crate with EventSink trait

Create Cargo workspace at v2/ level with members: src-tauri,
bterminal-core, bterminal-relay. Extract PtyManager and SidecarManager
into shared bterminal-core crate with EventSink trait for abstracting
event emission. TauriEventSink wraps AppHandle. src-tauri pty.rs and
sidecar.rs become thin re-exports. Move Cargo.lock to workspace root.
Add v2/target/ to .gitignore.

v2/bterminal-core/src/event.rs

@@ -0,0 +1,5 @@
+/// Trait for emitting events from PTY and sidecar managers.
+/// Implemented by Tauri's AppHandle (controller) and WebSocket sender (relay).
+pub trait EventSink: Send + Sync {
+    fn emit(&self, event: &str, payload: serde_json::Value);
+}