Feature: Send messages to running agents without stopping them

Problem

When agents are running (spawned via Task tool), the only options are:

• ctrl+c - Interrupt and stop agents (loses progress)
• ctrl+b - Run in background (no interaction possible)

There's no way to send a message to a running agent mid-execution, like you can in a normal Claude Code CLI chat.

Use Case

When reviewing a PR with multiple parallel agents:

⏺ Running 3 agents…
   ├─ dms-reviewer (Review DMS changes) · 8 tool uses
   ├─ mordor-reviewer (Review Mordor changes) · 5 tool uses
   └─ webapp-reviewer (Review Webapp changes) · 3 tool uses

I want to be able to type a message like:

> hey, also check the test files for coverage

And have the agents receive this as additional context/instruction without stopping them.

Current Workaround

• Interrupt, lose progress, re-spawn with updated prompt
• Or specify everything upfront (but you don't always know what you need until you see partial results)

Proposed Solution

Allow sending messages to running agents that get injected into their context, similar to how the main CLI chat works. Options could include:

1. Broadcast to all running agents
2. Send to a specific agent by name
3. Queue messages that agents receive at their next "checkpoint"

Environment

• Claude Code CLI
• Using Task tool to spawn parallel agents