add task input form and improve script runner output

- fetch task input definitions from API and render configurable inputs
- boolean inputs render as No/Yes toggle (e.g. delete_source)
- auto-filled inputs (file_path) are hidden from the form
- wrap script output in dark pre/code block with copy button
- fix ffmpeg -nostdin for batch directory processing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

seed/tasks/convert-to-mp3/TASK.md

@@ -26,6 +26,10 @@ inputs:
   file_path:
     type: string
     description: Path to an audio file or directory to convert.
+  delete_source:
+    type: boolean
+    description: Delete source files after successful conversion.
+    default: false
 args: [file_path]
 ---