Show active agent count in titlebar #401

Summary

Display a small badge/counter in the titlebar showing how many agents are currently active (working state) across all worktrees. Helps users know at a glance how many parallel tasks are in flight without scanning the sidebar.

Motivation

When running 3-4 agents across worktrees, you have to visually scan the sidebar to see who's working. A simple "3 agents active" indicator in the titlebar gives instant awareness.