[BUG] /plugin list returns empty output for plugins installed from third-party marketplace

Description

/plugin list returns no output even though plugins from a third-party marketplace are correctly installed and recorded in ~/.claude/plugins/installed_plugins.json.

Steps to Reproduce

1. Add a third-party marketplace (e.g. cyberuni) via /plugin marketplace add.
2. Install a plugin from it: /plugin install <plugin>@cyberuni.
3. Run /plugin list.
4. Observe: no output — the installed plugins are not listed.

Expected Behavior

/plugin list lists all installed plugins regardless of which marketplace they came from.

Actual Behavior

/plugin list returns (no content) even though ~/.claude/plugins/installed_plugins.json correctly contains the installed entries:

{
  "version": 2,
  "plugins": {
    "research-workbench@cyberuni": [
      {
        "scope": "user",
        "installPath": "/home/unional/.claude/plugins/cache/cyberuni/research-workbench/1.1.0",
        "version": "1.1.0",
        "installedAt": "2026-05-31T22:13:38.498Z",
        "lastUpdated": "2026-06-02T04:29:59.426Z"
      }
    ],
    "agent-initialization@cyberuni": [
      {
        "scope": "user",
        "installPath": "/home/unional/.claude/plugins/cache/cyberuni/agent-initialization/3e076f2b81c8",
        "version": "3e076f2b81c8",
        "installedAt": "2026-06-02T04:09:29.412Z",
        "lastUpdated": "2026-06-02T04:09:29.412Z"
      }
    ]
  }
}

Environment

• OS: Linux (WSL2) — Linux zeta 6.6.114.1-microsoft-standard-WSL2 x86_64
• Node: v24.15.0
• Claude Code: 2.1.160
• Marketplace: cyberuni (third-party, directory-based)

Notes

/plugin marketplace list correctly shows the cyberuni marketplace as configured. The data is present on disk; /plugin list appears to filter or skip plugins whose marketplace key does not match claude-plugins-official.