Display a native OS notification when long running tasks complete

### Describe the feature or problem you'd like to solve

When long running tasks happen, it would be nice to be alerted when they have completed.

### Proposed solution

It will bring the users back into the copilot flow to continue their work.

### Example prompts or workflows

set notification time to 20 seconds
disable notifier
enable notifier

### Additional context

If copilot CLI is processing for X (user defined, default say 20) seconds, they get a notification native to their OS (Specifically Windows 11 is a toast notification, etc), once Copilot is waiting for their input again.

No files or tests are named. Start by tracing the Copilot CLI flow for long-running tasks and the configuration path for enabling, disabling, and setting the 20-second threshold. Done means a native OS notification appears when processing ends and the CLI is waiting for input again.

Written by the indexing model from the issue text.