fix: add path traversal guard in sync_marketplace_to_plugins.py

[xiaolai]

Automated audit: This PR was generated by NLPM, a natural language programming linter, running via claude-code-action. Please evaluate the diff on its merits.

The sync_marketplace_to_plugins.py hook script resolves the source field from marketplace.json into a file path but does not verify that the resolved path stays within the expected plugin root. A tampered marketplace.json could write files outside the repository. This PR adds a guard that skips any entry whose resolved path escapes the marketplace root.

• Added a check after plugin_dir = (marketplace_dir / source).resolve() that compares the resolved path against the marketplace root
• If the resolved path escapes the root, the entry is skipped with a warning to stderr and the loop continues

[fcakyon]

the check just compares text, so a folder named claude-settings-evil slips straight past it, and anyone who could edit marketplace.json could already write files anyway