Project-level enabledPlugins should auto-install (or prompt to install) plugins from built-in marketplaces

Problem

When a project .claude/settings.json declares enabledPlugins for plugins from the built-in claude-plugins-official marketplace, team members who open the project are not prompted to install them and don't see them in /plugins.

The current behavior:

• enabledPlugins only enables plugins that are already installed locally
• The install prompt only triggers for custom marketplaces declared in extraKnownMarketplaces
• Since claude-plugins-official is a built-in marketplace, redeclaring it in extraKnownMarketplaces has no effect

This means there's no way for a project maintainer to ensure team members get the right plugins without manually telling each person to run /plugin install <name>@claude-plugins-official.

Expected behavior

When a project .claude/settings.json has enabledPlugins entries, Claude Code should either:

1. Auto-install the plugins on session start (preferred), or
2. Prompt the user to install missing plugins (similar to how extraKnownMarketplaces works for custom marketplaces)

This should work for all marketplaces, including the built-in claude-plugins-official.

Example config

{
  "enabledPlugins": {
    "superpowers@claude-plugins-official": true,
    "sentry@claude-plugins-official": true,
    "notion@claude-plugins-official": true
  }
}

A team member opening this project for the first time should be prompted: "This project uses plugins: superpowers, sentry, notion. Install them?"

Motivation

Project-scoped plugin configuration is a great feature for team alignment, but it's effectively broken for the most common case (official marketplace plugins). The workaround of messaging each team member to manually install is tedious and doesn't scale.