Related Issue

Resolve #2303 #1779

Description

Fixes the UserPromptSubmit hook payload so that the prompt field contains the actual user input text instead of an empty string.

Problem:
The shell UI passes user input as list[ContentPart] to soul.run(). The existing code only checked isinstance(user_input, str), causing text_input_for_hook to always fall back to "". As a result:

• The prompt field in the UserPromptSubmit hook payload was always empty.
• Regex-based matcher filters on prompt text never matched.

Fix:
When user_input is not a string, extract the text from the ContentPart list using the same Message.extract_text() approach already used by the agent turn loop:

if isinstance(user_input, str):
    text_input_for_hook = user_input
else:
    user_message = Message(role="user", content=user_input)
    text_input_for_hook = user_message.extract_text(" ").strip()

This ensures plain-text input (and any text extractable from multimodal input) is correctly passed to the hook engine for both payload generation and matcher evaluation. As a result, the payload.prompt can contain the message sent by the user (or adjusted message when SubagentStart is called).

Checklist

• I have read the CONTRIBUTING document.
• I have linked the related issue, if any.
• I have added tests that prove my fix is effective or that my feature works.
• I have run make gen-changelog to update the changelog.