Unified script model and script-aware execution

Fabrik now uses a single script model, rule = "script", with a shared [target.script] configuration for both inline scripts and annotated script files. The fabrik run and fabrik exec commands are now script-aware, executing scripts directly via their shebang or auto-detecting the runtime when you run fabrik exec -- <runtime> <script>. The documentation and website have also been refreshed around script files and script rules, and older task and runtime-script guidance has been removed.