Adds /history [n] to display the last N user/assistant messages from
the current session (default 10, max 50).
- Tool and system messages are filtered out for readability
- Long messages are truncated to 200 characters with an ellipsis
- Multimodal content (image blocks) is collapsed to its text parts
- Invalid count argument returns a usage hint
- /history n uses prefix routing; /history uses exact routing
Also registers /history in build_help_text().