feat(Handler/ext-host-proxy): Add handler for proxying extension host API calls

- Implements new proxy handler in Mountain backend to forward extension host requests to Cocoon sidecar process via Vine IPC
- Creates basic scaffolding for RPC communication between Sky UI and extension host process
- Uses placeholder process ID (1234) temporarily until dynamic process management is implemented
- Includes initial argument handling that will be expanded with structured API call validation
- Prepares ground for running VS Code extensions by enabling command proxying through Track dispatcher
- Required for MVP Path A strategy to reuse existing extensions via Cocoon shim layer
- TODOs indicate future work needed for production readiness (process ID management, payload validation)

Author: NikolaRHristov

Source/handlers/proxy.rs

@@ -0,0 +1,22 @@
+use serde_json::{json, Value};
+use tauri::{AppHandle, Runtime, Window};
+use crate::vine; // Use the Vine IPC layer
+
+pub async fn handle_ext_host_proxy<R: Runtime>(
+    _app: AppHandle<R>,
+    _window: Window<R>,
+    args: Vec<Value>,
+) -> Result<Value, String> {
+    println!("[Proxy Handler] Proxying call to Cocoon: {:?}", args);
+    // TODO: Identify target Cocoon process instance
+    let target_process_id = 1234; // Placeholder
+
+    // TODO: Robust arg handling - expect specific structure for API call
+    let request_payload = args.get(0).cloned().unwrap_or(json!(null));
+
+    // Use Vine to send the request and await response
+    match vine::send_to_sidecar(target_process_id, request_payload).await {
+        Ok(response) => Ok(response),
+        Err(e) => Err(format!("Cocoon IPC Error: {}", e)),
+    }
+}