Feature request: automatic plugin updates

Problem

Plugins installed from marketplace repos are cached at a specific git commit SHA and version string. They never update unless the user manually runs claude plugin update <name>. This creates several issues:

• Team members run stale plugin versions without realizing it
• Plugin authors push fixes/features that don't reach users for days or weeks
• /reload-plugins reloads from cache — it doesn't pull new versions
• If the marketplace clone gets corrupted or deleted, the plugin silently stops loading with no actionable error

Proposed solution

Add an autoUpdate option, either:

Per-plugin during install:

claude plugin install my-plugin --auto-update

Global setting in settings.json:

{
  "plugins": {
    "autoUpdate": true,
    "autoUpdateIntervalHours": 24
  }
}

Per-marketplace in extraKnownMarketplaces:

{
  "polyai-marketplace": {
    "source": { "source": "github", "repo": "org/repo" },
    "autoUpdate": true
  }
}

Expected behavior

• On session start (or at a configurable interval), check if the installed version/SHA matches the latest on the marketplace repo's default branch
• If not, pull the latest version into cache automatically
• Show a one-line notice: Updated plugin "my-plugin" (0.1.0 → 0.1.1)
• Respect an opt-out flag for users who want manual control

Context

• The CLI itself already has autoUpdatesChannel for self-updates — this would extend the same concept to plugins
• Org-managed plugins especially benefit, since admins push updates expecting them to propagate without user intervention
• Currently the only workaround is CI or shell hooks that run claude plugin update on a schedule