Workspace agent plugin recommendations are ignored in VS Code

### Description

VS Code does not surface agent plugins configured as workspace recommendations in settings.json.

This affects both:

- A plugin from a workspace-local marketplace.
- A public plugin from the built-in `awesome-copilot` marketplace.

The same workspace-local configuration works correctly in GitHub Copilot CLI. For CLI, the plugin is enabled in sessions within the workspace and disabled for the rest.

### Environment

- Copilot Chat Extension Version: `0.59.2026072404`
- VS Code Version: `1.131.0-insider`
- OS Version: Windows
- Feature (e.g. agent/edit/ask mode): Agent plugin workspace recommendations
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): Not model-specific
- Logs: No relevant warning or error appears in the Copilot Chat debug log.

### Workspace configuration

settings.json:

```json
{
"extraKnownMarketplaces": {
"example-internal-marketplace": {
"source": {
"source": "directory",
"path": "."
}
}
},
"enabledPlugins": {
"example-plugin@example-internal-marketplace": true,
"testing-automation@awesome-copilot": true
}
}
```
### Local marketplace structure

```text
.github/
copilot/
settings.json
plugin/
marketplace.json
plugins/
example-plugin/
.claude-plugin/
plugin.json
skills/
example-skill/
SKILL.md
```

marketplace.json:

```json
{
"name": "example-internal-marketplace",
"metadata": {
"description": "Internal workspace plugins",
"version": "1.0.0",
"pluginRoot": "./.github/plugins"
},
"plugins": [
{
"name": "example-plugin",
"source": "example-plugin",
"description": "Example internal plugin",
"version": "1.0.0"
}
]
}
```

Plugin manifest:

```json
{
"name": "example-plugin",
"description": "Example internal plugin",
"version": "1.0.0",
"author": {
"name": "Example Author"
}
}
```

### Steps to reproduce

1. Open the trusted repository in VS Code Insiders.
2. Add the workspace configuration shown above.
3. Reload the VS Code window.
4. Send a message in Copilot Chat.
5. Open the Extensions view.
6. Search for:

```text
@agentPlugins @recommended
```

7. Search for:

```text
@agentPlugins
```

### Expected behavior

According to the workspace plugin recommendation documentation:

- VS Code displays a notification after the first chat message.
- Both configured plugins appear under `@agentPlugins @recommended`.
- The public `testing-automation@awesome-copilot` plugin is recommended even if the local marketplace cannot be resolved.
- The user can review and install or enable the recommendations.

### Actual behavior

- No recommendation notification appears.
- `@agentPlugins @recommended` shows neither plugin.
- `testing-automation` is available from `awesome-copilot`, but it is not marked as recommended.
- Reloading the VS Code window does not change the behavior.
- No visible validation error or warning explains why the workspace configuration was ignored.

### GitHub Copilot CLI comparison

The repository-local marketplace and plugin configuration work correctly in GitHub Copilot CLI. The CLI recognizes the marketplace and plugin, suggesting that their manifests are structurally valid.

The failure of the public `testing-automation@awesome-copilot` control indicates that the VS Code issue is not limited to local `directory` marketplace resolution. It appears that the workspace `enabledPlugins` recommendations are not being processed at all.

### Questions

1. Is settings.json currently supported for workspace plugin recommendations in this VS Code Insiders build?
2. Are additional settings or feature flags required for `enabledPlugins` to be processed?
3. Should `enabledPlugins` recommend plugins that are available but not yet installed?