[BUG] Parallel Explore agents hang at Creating/Harmonizing step - no timeout

Bug Description

When launching multiple parallel Explore agents, Claude Code hangs indefinitely at the "Creating..." or "Harmonizing..." step after agents complete their work. No timeout or stuck detection.

Environment

• Claude Code Version: 2.1.22
• OS: macOS Darwin 25.2.0
• Model: Opus 4.5

Reproduction Steps

1. Start Claude Code session
2. Ask Claude to launch 3 parallel Explore agents (e.g., investigating different parts of a codebase)
3. Agents complete their tool calls (Glob, Grep, Read) successfully
4. UI shows "Creating..." or "Harmonizing..." step with spinner
5. Hangs indefinitely - no progress, 1.3k tokens shown, "thought for 3s"

Debug Log Evidence

06:52:39 - Agents doing tool calls (Glob, Grep, Bash)
06:52:41 - Last PostToolUse hook for Grep
06:52:41+ - Only "High write ratio" logs, no further tool activity
         - No SubagentStop event logged
         - Session stuck at "Creating/Harmonizing" step
         - Required manual interrupt (esc) after 2+ minutes

Screenshot

The UI shows 3 Explore agents completed (showing tool uses and token counts), but the main session is stuck at "Creating..." for extended periods.

Additional Context

• Also observing built-in cloud MCP servers (claude.ai Slack, Atlassian, Microsoft 365, monday.com) failing authentication on every startup
• These servers cannot be disabled via user mcp.json config
• Unclear if these auth failures contribute to the hang

Related Issues

• #4580 - Claude Code freezes during multi-agent task JSON serialization (locked)
• #15945 - MCP Server causes 16+ hour hang

Expected Behavior

• Timeout/stuck detection should kick in
• Result synthesis step should complete or fail gracefully
• Option to disable built-in cloud MCP servers that require auth

Workaround

Running Explore agents sequentially instead of in parallel avoids the hang.