AI Staging: JSON decoding error 'Failed to decode action envelope' with starter prompt

[aixxo]

When using AI Staging with the starter prompt, the app fails with the following error:

Failed to decode action envelope: DecodingError.dataCorrupted: Data was corrupted. Debug description: The given data was not valid JSON.. Underlying error: Error Domain=NSCocoaErrorDomain Code=3840 "Unexpected character 's' after top-level value around line 1, column 5." UserInfo={NSJSONSerializationErrorIndex=4, NSDebugDescription=Unexpected character 's' after top-level value around line 1, column 5.}

Steps to reproduce:

1. Open BetterStage
2. Go to AI Staging
3. Use the default/starter prompt
4. Trigger the action

Observed behavior:
The action envelope returned by the model fails JSON decoding — the underlying NSJSONSerializationErrorIndex=4 / "Unexpected character 's' after top-level value around line 1, column 5" suggests the AI response is not valid JSON (e.g. it may contain a stray non-JSON token such as the start of a second value or trailing text right after the first JSON value ends).

Expected behavior:
AI Staging should either produce valid JSON reliably, or handle/report malformed model output gracefully instead of erroring out on the raw decode.