CLI: Review which commands should display update notifications

## Summary

After #17350 changed update notifications to opt-in, we should review which commands display the notification and establish a clear rule for when it's appropriate.

## Current state

Commands that currently opt in to `UpdateNotificationsEnabled`:

| Command | Purpose |
|---------|---------|
| AddCommand | Add integrations to a project |
| DashboardRunCommand | Run the Aspire dashboard |
| InitCommand | Initialize an Aspire project |
| LsCommand | List Aspire projects |
| NewCommand / TemplateCommand | Create a new Aspire project |
| PipelineCommandBase (Deploy/Destroy/Do/Publish) | Deploy or publish |
| RestoreCommand | Restore project dependencies |
| RunCommand (non-detach) | Run an AppHost interactively |
| SetupCommand | Set up the Aspire layout |
| StartCommand | Start an AppHost in the background |
| StopCommand | Stop a running AppHost |

Commands that were removed from opt-in during review:
- DescribeCommand, ExportCommand, LogsCommand, PsCommand, ResourceCommand, WaitCommand

## Proposed rule

> **Update notifications should only be displayed for project-lifecycle commands** — commands that create, configure, build, run, or deploy an AppHost. Commands that interact with individual resources within a running AppHost, produce machine-readable output, or are quick utilities should NOT display update notifications.

Rationale:
- Project-lifecycle commands (new, init, run, start, deploy) are natural moments for users to learn about updates
- Resource-interaction commands (logs, ps, describe, wait, resource start/stop) are executed frequently during development and the notification becomes noise
- Machine-readable/streaming commands should never show notifications as they pollute structured output

## Action items

- [ ] Confirm the proposed rule matches team intent
- [ ] Evaluate whether `LsCommand` and `StopCommand` should still show notifications (they're quick commands)
- [ ] Add the rule to agent instructions so future commands follow the convention