feat(golemcore/nextcloud): add Nextcloud WebDAV file plugin

[golemcore1]

Summary

• add a new first-party golemcore/nextcloud plugin module backed by the standard Nextcloud WebDAV files endpoint
• expose a nextcloud_files tool with full file-layer operations: list_directory, read_file, write_file, create_directory, delete, move, copy, and file_info
• support both inline UTF-8 text reads and binary file attachments, plus base64 writes for arbitrary file content
• add dashboard settings for Nextcloud connection, root sandbox, operation policy flags, and a Test Connection action
• register the module in the reactor, add initial marketplace registry metadata, and document the plugin in the repository README
• cover the implementation with mocked service, provider, settings, wiring, and WebDAV client tests