Feature: Visual progress indicators for background subagents

Feature Request

When using the Agent tool with run_in_background: true to dispatch multiple subagents in parallel, there's no visual feedback on their progress in the terminal.

Current behavior

• A spinner shows the current foreground task
• Background agents show nothing until they complete with a <task-notification>
• The only way to check progress is manually reading output files via tail -f

Proposed behavior

A compact, real-time status panel showing all running background agents:

┌─ Agents ──────────────────────────────────────┐
│ ⠋ Update service files          [2m 14s]      │
│ ⠙ Create billing module         [0m 48s]      │
│ ✓ Install dependencies          [0m 12s] DONE │
│ ✓ Write security rules          [0m 08s] DONE │
└───────────────────────────────────────────────┘

Why this matters

When executing implementation plans with subagent-driven development, it's common to dispatch 3-5+ agents in parallel. Without visual feedback, the user has no sense of progress and the terminal feels "stuck" during long waits.

Even a minimal version (agent name + elapsed time + spinner) would be a huge UX improvement for power users running parallel workloads.

Context

Discovered while running 30+ subagents in a single session to build a SaaS platform migration (7 implementation plans, all parallelized). The lack of visual feedback was the main friction point.