Allow opening a new empty chat directly from the Agents view

Feature request

In the Agents view (claude agents / the agents list UI), there's no way to open a brand new, empty chat session. The only way to start a new agent/chat from that view is to type and send a message immediately.

This makes several common actions unnecessarily clunky, because they require a message to already exist before you can act:

• Running /mcp or other slash commands on a fresh session. You have to send a throwaway message first, enter the chat, then run /mcp — there's no way to land in a clean chat and just run a command.
• Starting a chat on a different model. Today the flow is: send a message with the default model → enter the chat → realize/decide you want a different model → change the model → the original message is now stale/wrong context → stop generation → re-send the message under the new model. What should be a single step (pick model, then chat) becomes a multi-step workaround.
• More generally, any setup step you'd want to do before the first real message (switching model, checking /mcp server status, checking /config, etc.) currently forces you to burn a message and interrupt/stop it first.

Suggested fix

Either of these would solve it:

1. Let the Agents view open a new chat with no initial message (an empty session you can then run commands in or configure before typing anything), or
2. Let the Agents view accept a command as the "first action" instead of requiring a message — e.g. launching straight into /mcp or with a model already selected, without needing to send-then-stop a message first.

Why this matters

Right now, any pre-chat setup (model selection, /mcp, /config, etc.) requires sending a message you don't want, entering the chat, stopping it, fixing the setting, and resending — extra steps for something that should be zero-cost.