Discover tab shows plugins that are already installed at project scope

Description

When plugins are installed with project scope (recorded in .claude/settings.json under enabledPlugins), they still appear in the /plugin Discover tab. Expected behavior is that already-installed plugins should be hidden from Discover results.

Steps to Reproduce

1. Install multiple plugins at project scope (they appear in .claude/settings.json enabledPlugins)
2. Run /plugin and open the Discover tab
3. Observe that installed plugins still appear in the list

Expected Behavior

Plugins already installed in the current project should not appear in Discover results (or should be clearly filtered out by default).

Actual Behavior

All marketplace plugins appear in Discover regardless of installation status at project scope, making it difficult to find new/uninstalled plugins.

Related Issues

• #19743 — Discover hides plugins installed at project scope in other projects
• #28739 — Discover tab hides locally-scoped plugins installed in other projects
• #14185 — Project-scoped plugins: Discovery tab incorrectly hides plugin

These issues describe the inverse problem (plugins hidden when they shouldn't be). This issue is about plugins shown when they should be hidden — both stem from the Discover tab's filtering logic not correctly handling project-scoped installations.

Environment

• Claude Code CLI
• Plugins installed via project-level .claude/settings.json

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