Show MCP server failures in the UI instead of debug log

MCP server spawn failures were going to dbglog where the user
wouldn't see them. Route through the agent's notify so they appear
on the status bar.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/tools/mod.rs

@@ -164,7 +164,7 @@ pub async fn dispatch_with_agent(
                 None => true,
             };
             if allowed {
-                if let Ok(result) = mcp_client::call_tool(name, args).await {
+                if let Ok(result) = mcp_client::call_tool(name, args, agent.as_ref()).await {
                     return result;
                 }
             }