Feature Request: Auto-update plugins (all or per-plugin)

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

Currently, when plugins in the marketplace are updated, users must manually check for and apply updates. This creates friction. Users may run outdated versions with bugs or missing features without realizing it, and plugin authors have no way to ensure users are on the latest version.

### Proposed solution

Add an auto-update mechanism for installed plugins with the following capabilities:

1. **Global auto-update toggle**: a setting (e.g., `copilot.plugins.autoUpdate: true`) that automatically pulls the latest version of all installed plugins on a configurable cadence (e.g., on session start or daily).
2. **Per-plugin granularity**: allow users to pin specific plugins to a version or opt individual plugins in/out of auto-update (e.g., `copilot.plugins.autoUpdateExclude: ["plugin-a"]`).
3. **Update notifications**: when an update is applied (or available, if auto-update is off), surface a brief notification with a changelog summary so users are aware of what changed.
4. **Rollback support**: if an auto-updated plugin causes issues, provide a quick way to revert to the previous version.

### Example prompts or workflows

_No response_

### Additional context

_No response_

No files, tests, or entry points are named. Start by locating the plugin marketplace, installation and update flows, configuration handling, notification system, and version storage. Define how global and per-plugin settings, changelog notifications, and rollback should work, then identify tests covering each workflow.

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