fix: Ensure AI doesn't return json markdown (#3809)

## Description

Detect route started returning json in markdown instead of pure json
`json\n[\"generate-template-prompt\"]\n`

## Steps for reproduction

1. click button
2. expect xyz

## Code Review

- [ ] hi @kof, I need you to do
  - conceptual review (architecture, feature-correctness)
  - detailed review (read every line)
  - test it on preview

## Before requesting a review

- [ ] made a self-review
- [ ] added inline comments where things may be not obvious (the "why",
not "what")

## Before merging

- [ ] tested locally and on preview environment (preview dev login:
5de6)
- [ ] updated [test
cases](https://github.com/webstudio-is/webstudio/blob/main/apps/builder/docs/test-cases.md)
document
- [ ] added tests
- [ ] if any new env variables are added, added them to `.env` file

packages/ai/src/chains/command-detect/command-detect.system.prompt.md

@@ -9,3 +9,5 @@ Avaliable tasks are provided below as an object with task_name:task_description
 The task description can help you infer the task name to pick. For example if the user is asking to translate you should respond with ["copywrite"]
 
 Respond with a valid JSON array of task names that are relevant for the user request. Start with [
+
+Do not start your response with ```json