Agents window needs CLI/native Chat feature parity for daily use

The dedicated Agents window has better ergonomics than the CLI, but it currently lacks too many basic capabilities that exist in Copilot CLI or the normal Copilot Chat view:

- Attach entire folders or additional directories reliably
- Support multi-root and cross-folder context
- Provide slash commands for model, reasoning level, session management, directory management, and related actions
- Provide keyboard shortcuts for switching agents, models, reasoning effort, and sessions
- Provide a clear session list/history with familiar session controls
- Expose the active model, agent, reasoning level, working directory, and context in an obvious way
- The result is that the dedicated UI is ergonomically attractive but functionally much less capable than the CLI. Users have to abandon the UI for basic workflows that are already supported elsewhere.

This should be treated as a parity requirement, not as a collection of unrelated polish issues. Either CLI commands should work in the Agents window, or every command should have an obvious UI and command-palette/keybinding equivalent.