[FEATURE] Display active subagent count in the CLI status line

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When Claude Code spawns multiple subagents via the Task tool to work in parallel, there is no real-time visibility into how many agents are currently active. The CLI status line already displays useful session metrics (context window usage, token count), but there is no equivalent indicator for concurrent subagent activity.

This creates a blind spot during complex, multi-step workflows where parallel delegation is happening. As a user, I can see tokens being consumed and context filling up, but I have no idea whether 1 or 5 subagents are running behind the scenes, whether they've completed, or whether the main agent is waiting on them. The only current option is Ctrl+T to toggle a task list, which requires an explicit action and shifts focus away from the conversation.

Proposed Solution

Add a persistent active subagent counter to the CLI status line, displayed alongside the existing context window and token metrics. For example:

Tokens: 12.4k / 200k   Context: 34%   Agents: 3/3 running

When no subagents are active, the indicator would either hide or show Agents: 0. As agents complete, it would update in real time:

Agents: 2/4 running  (2 completed)

This should be:

• Always visible in the status line — no keypress required
• Lightweight — just a counter, not a full task list
• Consistent with the existing status line design language (compact, informative, non-intrusive)

Alternative Solutions

• Ctrl+T task list toggle: Already exists but requires explicit user action and takes up screen space. Good for detailed inspection, but not for passive awareness.
• Agent Teams with tmux split panes: Provides full visibility but is an experimental feature that fundamentally changes the interaction model. Overkill for users who just want a quick "how many agents are working right now?" glance.
• Log output per agent: Would clutter the main conversation thread and defeat the purpose of subagents isolating verbose work.

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI)

Use Case Example

1. I ask Claude Code to refactor a module that involves researching the codebase, running tests, and checking documentation
2. Claude spawns 3 parallel subagents (one exploring files, one running tests, one fetching docs)
3. Currently: I see the main conversation paused with no indication of background work — it feels like it's "stuck"
4. With this feature: I glance at the status line and see Agents: 3/3 running, giving me confidence that work is happening in parallel
5. As agents finish, the counter updates: Agents: 1/3 running (2 completed) — I know results are about to come in
6. All agents complete: counter resets or hides — I know the main agent is now synthesizing results

Additional Context

This is inspired by how other developer tools handle background process visibility:

• Docker Desktop shows running container counts at a glance
• VS Code shows background task counts in the status bar (formatters, linters, language servers)
• tmux shows window/pane counts persistently

The status line in Claude Code already follows this pattern for tokens and context — extending it to subagents would be a natural, consistent addition. This would particularly benefit users running complex workflows where Claude leverages parallel subagents heavily, improving trust and perceived responsiveness without adding UI complexity.