Add send_chat + get_messages tools for Soulseek private messages

[lesteak]

What

Adds two MCP tools for Soulseek private messaging:

• send_chat(username, message) — send a private chat message to a user (via aioslsk PrivateMessageCommand).
• get_messages(clear=True) — return private messages received this session (newest last); buffered by a PrivateMessageEvent listener registered at login (last 200, configurable via SLSK_MESSAGE_BUFFER).

Why

Some peers run anti-leech bots that gate downloads behind a private-chat reply. These tools let an agent read the challenge (get_messages) and respond (send_chat) to unblock the queue, instead of the download silently stalling.

Notes

• No new dependencies — uses aioslsk's existing PrivateMessageCommand / PrivateMessageEvent.
• The inbound listener is wrapped so a handler error can't break the client.
• Mirrors the existing tool/wrapper pattern (peer_status); uv run confirms both tools register alongside the existing eight.