Extends the same `~/.agents/skills` special case that
`create_directory`, `edit_file`, and `write_file` already use to the
path tools, so the agent can copy or move skills into and out of the
global skills folder and delete individual skills or skill resources
beneath it.

- `copy_path` now allows source and/or destination to be a descendant of
`~/.agents/skills`, going through `fs::copy_recursive` directly when one
side is outside the project.
- `move_path` now allows source and/or destination to be a descendant of
`~/.agents/skills`, going through `fs.rename` directly when one side is
outside the project. Moving the `~/.agents/skills` root itself is
rejected.
- `delete_path` now allows deleting any file or directory beneath
`~/.agents/skills`, going through `fs.remove_dir` / `fs.remove_file`
directly. Deleting the `~/.agents/skills` root itself is rejected.
- All three tools still always prompt for approval on agent-skill paths,
even when default tool permissions are set to allow.
- Added shared helpers in `tool_permissions.rs` for resolving global
skill descendants and rejecting operations on the skills root where
needed.
- Added tests covering copying and moving skills in both directions,
deleting a global skill directory/file, and rejecting deletion of the
skills root.

Release Notes:

- Agent can now copy or move skills into and out of `~/.agents/skills`
and delete individual skills, with an explicit confirmation prompt for
each operation