Feat request: /plugin update-all command

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

a /plugin update-all command would be useful.

### Proposed solution

The CLI currently only supports the /plugin interactive command, which updates plugins one at a time. There's no built-in "update all" option or batch command.

Workaround — you can bulk-update by editing the config directly:

1. Delete the cached plugin folders: Remove-Item ~\.copilot\installed-plugins\ic3-ai-plugins\* -Recurse -Force
2. Clear the ic3-ai-plugins entries from ~\.copilot\config.json — remove all objects in installed_plugins where marketplace is "ic3-ai-plugins"
3. Run /plugin in the CLI, select ic3-ai-plugins, and reinstall all

Or more practically — run /plugin and update them one by one. It's tedious with 21 plugins, but it's the supported path today.

You could also file this as a feature request via /feedback — a /plugin update-all command would be useful.

### Example prompts or workflows

_No response_

### Additional context

_No response_