[BUG] Installing one plugin from marketplace.json loads ALL plugins

Description

When installing a specific plugin from a marketplace that has multiple plugins defined in marketplace.json, ALL plugins from that marketplace get loaded, not just the one being installed.

Reproduction Steps

1. Install document-skills@anthropic-agent-skills (contains xlsx, docx, pptx, pdf skills)
2. Install example-skills@anthropic-agent-skills (contains algorithmic-art, frontend-design, etc.)
3. Run /skills or check available skills
4. Observe duplicate skills: both document-skills:xlsx AND example-skills:xlsx exist, even though xlsx should only be in document-skills

Expected Behavior

Only the skills defined in the installed plugin should be registered. Installing document-skills should only register xlsx, docx, pptx, pdf. Installing example-skills should only register the 12 skills it declares.

Actual Behavior

Both installations load ALL plugins from the shared marketplace.json, resulting in:

• document-skills:xlsx, document-skills:pdf, etc.
• example-skills:xlsx, example-skills:pdf, etc. (duplicates!)
• Plus claude-api:* skills from both

Root Cause Analysis

Both plugins reference the same GitHub repo (anthropics/anthropic-agent-skills) and the marketplace.json file contains ALL plugin definitions. When the plugin loader reads marketplace.json, it loads every plugin defined in it, regardless of which specific plugin was installed.

Files involved:

• /plugins/cache/anthropic-agent-skills/document-skills/98669c11ca63/.claude-plugin/marketplace.json
• /plugins/cache/anthropic-agent-skills/example-skills/98669c11ca63/.claude-plugin/marketplace.json

Both marketplace.json files are identical and contain:

{
  "plugins": [
    {"name": "document-skills", "skills": [...]},
    {"name": "example-skills", "skills": [...]},
    {"name": "claude-api", "skills": [...]}
  ]
}

Suggested Fix

The plugin loader should only load the plugin matching the installed plugin name from marketplace.json, not all plugins defined in it.

Environment

• Claude Code: latest
• Platform: macOS