Undelete delete browsers from Invocation cancellation (#16)

<!-- mesa-description-start -->
## TL;DR

Restores the logic to delete browsers when an invocation is cancelled.

## Why we made these changes

This fixes a regression where cancelling an invocation would leave
orphaned browser instances running, consuming unnecessary resources.
This change ensures proper resource cleanup.

## What changed?

- In `cmd/invoke.go`, the invocation cancellation handler now calls
`client.Invocations.DeleteBrowsers` with a 30-second timeout to ensure
associated browsers are properly terminated.

<sup>_Description generated by Mesa. [Update
settings](https://app.mesa.dev/onkernel/settings/pull-requests)_</sup>
<!-- mesa-description-end -->

Author: hiroTamada

cmd/invoke.go

@@ -110,6 +110,9 @@ func runInvoke(cmd *cobra.Command, args []string) error {
 			); err != nil {
 				pterm.Error.Printf("Failed to mark invocation as failed: %v\n", err)
 			}
+			if err := client.Invocations.DeleteBrowsers(context.Background(), resp.ID, option.WithRequestTimeout(30*time.Second)); err != nil {
+				pterm.Error.Printf("Failed to cancel invocation: %v\n", err)
+			}
 		})
 	})