fix(cli): degrade to no-proxy on spawn error, and stop readStdin hanging on a TTY

A spawn 'error' arrives asynchronously, so the caller's try/catch never saw it
and a wrong-arch or non-executable proxy binary killed the CLI before the agent
launched. readStdin now rejects when stdin is a TTY instead of waiting forever.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SqmbqWDtNexzDfH5V3BMRz