github mobile notifications for remote sessions

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

would like to be notified when copilot cli requires my attention or when it completed a task.

### Proposed solution

will improve productivity.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start by tracing how Copilot CLI represents remote sessions and how it detects that a task needs attention or has completed. Then identify the project entry points for notification delivery and define the mobile notification workflow before implementation; done means both event types reliably reach the user's mobile device.

索引モデルが issue の本文から書いたものです。