No visibility into background subagent activity — session looks idle while agents work

Version: 2.1.170 (macOS)

Problem

When the assistant dispatches background subagents (Agent tool with run_in_background: true), the main session UI gives no indication that anything is happening. During a long multi-agent workflow (a design-system migration with implementer + reviewer subagents running 5-20 minutes each), the session repeatedly looked completely idle — no spinner, no progress line, nothing distinguishing "agents are working in the background" from "Claude is waiting for your input."

As a user I genuinely could not tell if the tool was doing anything or stuck waiting on me, multiple times, and had to ask "is it still working?" The assistant could only prove progress by running git status to show files changing underneath.

Expected

Some persistent, glanceable indicator for in-flight background tasks — e.g. a status-line segment or footer chip like ⠹ 1 background agent running (12m) — ideally with the task description, so a session with active background work never renders as idle.

Context

This was a session orchestrating subagent-driven development (multiple sequential background agents). The completion notifications work fine; it's the silence between dispatch and completion that's the problem.