Merge upstream/development: save_plugin_config JSON fix + plugins-list-header-buttons slot

Conflicts resolved: - plugins/memory/plugin.yaml: keep per_agent_config: true (intentional) - python/helpers/plugins.py: keep determined_toggle_from_paths + always_enabled + specific_overrides from our consolidation - python/helpers/files.py: take upstream's cleaner exists() guard Co-authored-by: Cursor <cursoragent@cursor.com>