Add Cursor Agent support

[TimofeySukh]

It would be useful to use Cursor Agent as another harness in moi alongside Claude Code, Codex, OpenClaw, and Hermes.

Cursor Agent already has a headless CLI with stream-json output, resumable sessions, model selection, and authentication commands, so it looks like a good fit for the existing harness interface.

A first experimental version could:

• show Cursor Agent as an option during workspace setup
• detect the cursor-agent executable and authentication state
• start and resume chats in the workspace directory
• stream assistant messages and tool calls into moi
• expose model selection when available

Relevant Cursor CLI docs:

• https://docs.cursor.com/en/cli/reference/parameters
• https://docs.cursor.com/en/cli/reference/output-format