[BUG] Multiple parallel foreground agents freeze after ~15-30 min, unblock on ctrl+b (move to background)

Description

When launching 3-4+ parallel Agent tool calls (foreground, no run_in_background), all agents freeze after approximately 15-30 minutes. Token consumption stops, tool calls show as active but make no progress (e.g., stuck on "Web Search", "Fetch", "Bash").

Switching agents to background with ctrl+b immediately unblocks them and they resume normal execution.

Reproduction

1. Launch 3-4 Agent tool calls in a single message (foreground, parallel)
2. Each agent performs web searches, fetches, and bash commands
3. Wait ~15-30 minutes
4. Observe: all agents stop progressing, token count frozen, last tool call stuck
5. Press ctrl+b to move agents to background
6. Observe: agents immediately resume and complete normally

Screenshot

Agents stuck for 27m33s with active tool calls showing no progress:

!Screenshot showing 4 parallel agents frozen

• Agent 1: "Web Search: Seth Godin..." — 18 tool uses, 27.2k tokens
• Agent 2: "Web Search: Polish small business..." — 17 tool uses, 24.8k tokens
• Agent 3: "Fetch: https://design.duolingo.com/writing/voice" — 15 tool uses, 25.9k tokens
• Agent 4: "Bash: Add Alex Hormozi content strategy..." — 32 tool uses, 39.9k tokens

All resumed immediately after ctrl+b.

Environment

• OS: Windows 10 Pro 10.0.19045
• Shell: bash (Git Bash)
• Model: Claude Opus 4.6 (1M context)
• Agent count: 4 parallel foreground agents
• Agent tools: WebSearch, WebFetch, Bash, Read, Write

Notes

• This is NOT specific to any particular MCP tool — happens with WebSearch, WebFetch, and Bash
• Previously observed with run_in_background: true as well (issues #20679, #17540, #20754), but the foreground variant is different — ctrl+b unblocks it
• Seems like the foreground polling/rendering loop blocks agent execution when multiple agents are active simultaneously
• Happens consistently with 3-4+ agents, unclear if 2 agents also triggers it

Workaround

Press ctrl+b to move agents to background when they freeze. They resume immediately.