feat(ui): a top-menu button showing which agents are working (replaces #518) #1471

Owner decision, 2026-09-27: replaces the always-on-top overlay (#518, reverted in #517 because a second always-on-top window broke macOS window behaviour).

Goal: a button in the app's top menu/title bar that shows at a glance which agents are working. For example, a count of busy agents, plus a dropdown listing each working agent with its state and quick navigation. It gives the at-a-glance status the overlay tried to give, without a second window.

Notes:

• Reuse the existing working-state source (the agent working-state / completion-indicator machinery) rather than a new tracker.
• No new always-on-top or panel-type window (see Floating always-on-top agent status overlay — reverted from main, revisit with a different window approach #518/revert: floating agent status overlay (#514, #516) #517 for why).
• Keyboard reachable, and consistent with the keyboard-first work (feat(ui): make every surface keyboard-operable with compact key hints and consistent chrome #1221) and the layer system (refactor(ui): named layers and one side-panel shell (#512) #1398).