feat(mcp): McpToolDescriptor is Copy (T10.4)

Needed so RmcpServer::tools() can hand back TOOL_DESCRIPTORS without a
manual field-by-field clone (next commit).

src-tauri/src/mcp/mod.rs

@@ -115,6 +115,7 @@ pub struct McpHandle {
     pub token: String,
 }
 
+#[derive(Debug, Clone, Copy)]
 pub struct McpToolDescriptor {
     pub name: &'static str,
     pub description: &'static str,