Discover tab shows project-scoped plugins as '(installed)' in unrelated projects

Bug Description

The Discover tab in /plugin marks plugins with a checkmark and "(installed)" even when they are only installed for a different project (project-scoped / local scope). This is confusing because:

• Installed tab: Correctly does NOT show the plugin (since you're in a different project)
• Marketplaces tab: Correctly does NOT list it under installed plugins
• Discover tab: Incorrectly shows it as "(installed)" with a green checkmark

Steps to Reproduce

1. Open project A (e.g., talent-cartogram)
2. Install a plugin with project/local scope (e.g., frontend-design)
3. Open a different project B (e.g., airwave)
4. Run /plugin → navigate to Discover tab
5. frontend-design shows as "(installed)" with a checkmark, even though it's not installed for project B

Root Cause

The Discover tab appears to check installed_plugins.json globally and marks any plugin as "installed" if it has ANY entry — regardless of scope or project path. It doesn't filter by the current project context.

Example from ~/.claude/plugins/installed_plugins.json:

"frontend-design@claude-plugins-official": [
    {
        "scope": "local",
        "projectPath": "/Users/p/Documents/Claude/talent-cartogram",
        ...
    }
]

When viewing from /Users/p/Documents/Claude/airwave, this plugin is not active, the Installed tab correctly excludes it, but Discover still shows it as installed.

Expected Behavior

The Discover tab should respect project scope: only mark a plugin as "(installed)" if it is installed for the current project (local scope matching current projectPath) or installed at user scope (globally).

Impact

This is confusing for users managing plugins across multiple projects. It makes it impossible to tell from the Discover tab which plugins are actually active in the current project vs. installed elsewhere.

Environment

• Claude Code v2.1.63
• macOS (Darwin)
• Opus 4.6 / Claude Max