fix(windows): use stdin for long prompts to avoid command line limit

[greg12579]

Summary

• Fixes agent crash on Windows when prompts exceed ~8191 character command line limit
• Prompts over 7000 characters are now passed via stdin instead of CLI argument
• Maintains compatibility with Claude Code's --print mode which accepts stdin input

Problem

On Windows, the command line has a length limit of ~8191 characters. When the wizard builds up conversation history, the prompt can exceed this limit, causing the agent process to fail immediately with exit code 1 and no output.

Symptoms:

• "Agent exited with code 1" error on second or subsequent wizard messages
• Process spawns but exits within ~30ms
• No error output from the agent itself

Solution

Detect when a prompt exceeds 7000 characters (leaving margin for command and args) on Windows, and instead of passing it as a CLI argument, write it to the process's stdin. Claude Code's --print mode accepts prompts via stdin, so this maintains compatibility.

Test plan

• First wizard message works (prompt under limit)
• Second wizard message works (prompt over limit, uses stdin)
• Verify other agents still work normally
• Test on macOS/Linux (should be unaffected)

🤖 Generated with Claude Code