[Bug] UserPromptSubmit hook event sends empty prompt field

Bug Description

The UserPromptSubmit hook event passes an empty string for the prompt field in the stdin JSON payload, regardless of what the user actually typed.

Steps to Reproduce

1. Configure a hook in ~/.kimi/config.toml:

[[hooks]]
event = "UserPromptSubmit"
command = "/path/to/capture-script.sh"
timeout = 10

2. Create a capture script that logs stdin:

#!/bin/bash
INPUT=$(cat)
echo "$INPUT" >> /tmp/kimi-hook-debug.log

3. Open Kimi CLI, type any message (e.g., "hello world"), and submit.

Expected Behavior

The hook should receive the user's actual prompt text:

{"hook_event_name": "UserPromptSubmit", "session_id": "...", "cwd": "/Users/...", "prompt": "hello world"}

Actual Behavior

The prompt field is always an empty string:

{"hook_event_name": "UserPromptSubmit", "session_id": "a072f750-4106-4abc-86cd-1728396cd387", "cwd": "/Users/linnian", "prompt": ""}

Impact

Third-party integrations that rely on hook events to track user prompts (e.g., Vibe Island) cannot read the prompt content, making prompt-related features non-functional.

Environment

• Kimi CLI version: latest (kimi-for-coding powered by kimi-k2.5)
• OS: macOS (Darwin arm64)
• Config format: ~/.kimi/config.toml with [[hooks]] array syntax

Start by locating the UserPromptSubmit hook dispatch and the code that constructs the stdin JSON payload; the issue does not name source files. Reproduce with the ~/.kimi/config.toml hook and capture script, then verify that the submitted text appears in the prompt field.