## Why
The app server should not need access to host skill root internals to decide
which paths require filesystem watching.
## What changed
- Add `HostSkillsService::watchable_skill_root_paths` to return non-plugin,
non-system skill roots for the watcher.
- Make `HostSkillRoot` and root resolution private to the skills extension.
- Update the app-server watcher to consume the filtered paths directly.
## Testing
Add a host service test confirming that user skill roots remain watchable while
plugin and generated system roots are excluded.
GitOrigin-RevId: b3e3b576e4ffe96230af71d9bf7c6c315b1705d3