Fix generated docs for nested plugin source paths

[kriptoburak]

Problem

The marketplace tooling derives each component directory from the plugin name instead of the marketplace source field.

The git entry declares "source": "./plugins/utilities/git", but:

• marketplace_update.py validate emits 13 false missing-file warnings.
• generated docs show paths under nonexistent plugins/git/ directories.
• generated Git command and Skill descriptions are blank because frontmatter is read from the wrong path.

Reproduction

1. Run python plugins/claude-plugin/skills/marketplace-update/marketplace_update.py validate.
2. Run python plugins/claude-plugin/skills/documentation-update/doc_generator.py.
3. Inspect the Git entries in docs/usage.md or docs/agent-skills.md.

Expected

Resolve local component files from each plugin’s declared source, and emit that source path in generated documentation. Add regression coverage for a plugin whose name differs from its directory.