feat: Show and apply plugin updates

[bojanrajkovic]

Summary

Show which installed plugins have updates available and allow updating them.

Motivation

Users should be able to see when newer versions of their installed plugins are available and easily update them.

Proposed Implementation

• Compare installed version against available version from marketplace
• Show update indicator in plugin list (e.g., [UPDATE] or ↑ icon)
• Add U key binding to update selected plugin (or all plugins with updates)
• Show changelog/release notes if available

Questions

• Does Claude CLI have an update command, or is it uninstall + reinstall?
• Should we support updating all plugins at once?

Complexity

Medium - Need to compare versions and potentially handle update logic.