Default save prompt and wrap assistant prose

[rowantrollope]

Summary

• Default the save-conversation prompt to no and show that default as Save this conversation? (y/N).
• Add a reusable default-answer path for y/n prompts while preserving existing behavior for callers without a default.
• Add configurable assistant prose wrapping via --wrap-cols N and DS4_AGENT_WRAP_COLS, defaulting to 100 columns.
• Clamp prose wrapping to the live terminal width when the terminal is narrower, while leaving code blocks, tool output, diffs, and command output unwrapped.

Validation

• make ds4-agent
• ./ds4-agent --help | rg wrap-cols
• Narrow pseudo-tty smoke test for terminal-width clamping and configured max-width wrapping