Reject "." as a plugin name — it resolves to the plugins directory
itself, which in force-install flows causes shutil.rmtree to wipe the
entire plugins tree.
- reject "." early with a clear error message
- explicit check for target == plugins_resolved (raise instead of allow)
- switch boundary check from string-prefix to Path.relative_to()
- add regression tests for sanitizer + install flow
Co-authored-by: Dusk1e <yusufalweshdemir@gmail.com>