refactor: extract configured group workspace observer

[teamleaderleo]

Problem

AppDelegate.swift contained ConfiguredGroupActionAsyncWorkspaceObserver, a standalone @MainActor observer that watches asynchronous workspace creation and joins the first new workspace to a group. It owned no AppDelegate state.

Change

Move the observer into Sources/App/ConfiguredGroupActionAsyncWorkspaceObserver.swift, preserving its static install/finish/dispose API, Combine subscription behavior, and lifecycle cleanup. Wire the new source into the Xcode project.

This PR is stacked on #71.

Validation

• swiftc -D DEBUG -parse Sources/App/ConfiguredGroupActionAsyncWorkspaceObserver.swift Sources/AppDelegate.swift
• git diff --check
• Confirmed the original type definition is removed from AppDelegate.swift.

A full xcodebuild remains blocked by the fork worktree's missing vendor/bonsplit/Package.swift dependency.

[teamleaderleo]

Consolidated into the coherent upstream slices #78 (UI-test infrastructure), #79 (session lifecycle), #80 (window ownership), and #82 (terminal diagnostics). The original branch remains available for history.