[feature] Asset Files

A workspace- and environment-scoped **Files** capability, alongside Knowledge Bases and Data Tables. Users can upload files (drag-and-drop or REST), browse and tag them, view/edit text-like files in a Monaco editor, preview images/PDFs, and download. Files can be user-uploaded or AI-generated, and carry a logical **format** (e.g. MARKDOWN, CSV, CHART) distinct from their MIME type.