[copyberry]

Load host skill roots through the skills extension

What changed

• Load ordinary recursive host skill roots with the skills extension's host loader.
• Keep plugin-specific roots on the existing loader so plugin snapshot caching and namespacing continue to work.
• Merge both kinds of root snapshots through the existing precedence and deduplication logic, including discovery paths and source filesystem mappings.
• Move HostSkillsSnapshot into the skills extension as the owner of host skill loading.

Testing

• Cover merged host and plugin catalogs, precedence for symlinked roots, discovery paths, and end-to-end rendering and invocation of host, repository, and plugin skills.