Multi Agent Support

The current implementation of Copilot features should be dropped. The new implementation for agents should feature multi agent support: Copilot CLI, Claude Code, Codex CLI, Grok Build, Antigravity CLI, OpenCode, Pi, etc.

There should be a sidebar panel that features sessions history for the current working directory. An agent picker to switch between CLI providers and a button to create a new session.

Session chats/windows should be opened in the main window or a new tab if tabs are enabled. Session windows should also support model picker from the chosen CLI provider.

There's no need for a 3 panel layout by default, right panel should be dropped. Obviously, the user can chose where to move the session history and session chat as with all current VS Code features.

The curent Copilot AI autocomplete can be dropped as well. Implement agent based features like sending code selection to agent for refactoring, `TO DO` comments as inline prompts for the agent, ask agent for a commit message, etc. All these features can be supported by multiple agents/harnesses.