document-skills plugin loads all skills from repo instead of only declared ones in marketplace.json

## Description

When installing the `document-skills` plugin via `/plugin install document-skills@anthropic-agent-skills`, **all 17 skills** from the repository are loaded, instead of only the **4 skills** declared in `marketplace.json`.

## Expected Behavior

According to `.claude-plugin/marketplace.json`, the `document-skills` plugin declares only 4 skills:

```json
{
"name": "document-skills",
"skills": [
"./skills/xlsx",
"./skills/docx",
"./skills/pptx",
"./skills/pdf"
]
}
```

Only these 4 skills should be available after installation.

## Actual Behavior

After installation, 17 skills are loaded under the `document-skills` namespace, including skills from `example-skills` and `claude-api` plugins:

- **Expected (4):** `xlsx`, `docx`, `pptx`, `pdf`
- **Also loaded (13 extra):** `algorithmic-art`, `brand-guidelines`, `canvas-design`, `claude-api`, `doc-coauthoring`, `frontend-design`, `internal-comms`, `mcp-builder`, `skill-creator`, `slack-gif-creator`, `theme-factory`, `web-artifacts-builder`, `webapp-testing`

## Root Cause

The install mechanism clones/caches the entire repository at a commit hash. The cached directory contains all skills from the repo under `skills/`. The plugin loader appears to load all skill directories found in the source path, without filtering against the `skills` array declared in `marketplace.json`.

Cache path example:
```
~/.claude/plugins/cache/anthropic-agent-skills/document-skills/{commit}/skills/
```

This directory contains all 17 skill folders, not just the 4 declared for `document-skills`.

## Steps to Reproduce

1. Run `/plugin install document-skills@anthropic-agent-skills`
2. Run `/skills` to list available skills
3. Observe 17 skills instead of 4

## Suggested Fix

The plugin loader should filter skill directories using the `skills` array from the matching plugin entry in `marketplace.json`, rather than loading all directories found in the source path.