Windows native crash when launching parallel subagents with local BYOK provider

## Summary

GitHub Copilot CLI crashed natively on Windows while using a BYOK/local OpenAI-compatible provider and launching multiple `general-purpose` subagents in parallel.

This was not a handled tool error; Windows Error Reporting recorded a native `BEX64` crash in `copilot.exe`.

## Environment

- Copilot CLI version: `1.0.45`
- Platform: Windows (`win32`, x64)
- OS version from CLI log: `10.0.26200`
- Node version from CLI log: `v24.15.0`
- Provider mode: custom/BYOK OpenAI-compatible provider
- Local provider: llama.cpp server at `http://localhost:8080/v1`
- Model: `Qwen_Qwen3.6-35B-A3B-Q4_K_M.gguf`
- Wire API: `completions`
- Prompt/context token limit configured: `120000`
- Working directory at the time: a non-git parent directory (`D:\Test\repos`)

## Windows Error Reporting details

From Windows Application event log:

```text
Faulting application name: copilot.exe, version: 1.0.45.0
Faulting module name: copilot.exe, version: 1.0.45.0
Exception code: 0xc0000409
Fault offset: 0x00000000021f2189
Faulting process id: 0x981C
Event Name: BEX64
Report Id: 22008bd5-251d-4655-be0b-bf50b162f35e
```

## Crash-adjacent Copilot log excerpt

The Copilot process log for PID `38940` ended immediately after dispatching three parallel `general-purpose` subagents:

```text
2026-05-12T00:38:56.756Z [DEBUG] Tool calls count: 3
2026-05-12T00:38:56.757Z [DEBUG] Running tool calls in parallel
2026-05-12T00:38:56.757Z [INFO] Task tool invoked with agent_type: general-purpose, name: Shader creation, description: Create D3D12 Pong shaders
2026-05-12T00:38:56.757Z [DEBUG] Task tool dispatch: agent_type="general-purpose", inputModel="(none)", resolvedModel="(none)", implicitModel="(none)", effectiveModel="(none, will use agent default)", sessionModel="Qwen_Qwen3.6-35B-A3B-Q4_K_M.gguf"
2026-05-12T00:38:56.759Z [INFO] Task tool invoked with agent_type: general-purpose, name: Engine core creation, description: Create D3D12 Pong engine core
2026-05-12T00:38:56.759Z [DEBUG] Task tool dispatch: agent_type="general-purpose", inputModel="(none)", resolvedModel="(none)", implicitModel="(none)", effectiveModel="(none, will use agent default)", sessionModel="Qwen_Qwen3.6-35B-A3B-Q4_K_M.gguf"
2026-05-12T00:38:56.759Z [INFO] Task tool invoked with agent_type: general-purpose, name: Pong game logic, description: Create Pong game logic
2026-05-12T00:38:56.759Z [DEBUG] Task tool dispatch: agent_type="general-purpose", inputModel="(none)", resolvedModel="(none)", implicitModel="(none)", effectiveModel="(none, will use agent default)", sessionModel="Qwen_Qwen3.6-35B-A3B-Q4_K_M.gguf"
2026-05-12T00:38:56.763Z [DEBUG] General-purpose agent: agentModelSetting="capi:Qwen_Qwen3.6-35B-A3B-Q4_K_M.gguf", currentModel="Qwen_Qwen3.6-35B-A3B-Q4_K_M.gguf", modelOverride="(none)", resolvedModel="Qwen_Qwen3.6-35B-A3B-Q4_K_M.gguf"
2026-05-12T00:38:56.764Z [INFO] General-purpose agent invoked with prompt: Create the D3D12 Neon Pong HLSL shader file at `D:\Test\repos\PongGame\shaders\Pong.hlsl`.
2026-05-12T00:38:56.764Z [DEBUG] General-purpose agent: agentModelSetting="capi:Qwen_Qwen3.6-35B-A3B-Q4_K_M.gguf", currentModel="Qwen_Qwen3.6-35B-A3B-Q4_K_M.gguf", modelOverride="(none)", resolvedModel="Qwen_Qwen3.6-35B-A3B-Q4_K_M.gguf"
2026-05-12T00:38:56.764Z [INFO] General-purpose agent invoked with prompt: Create a complete D3D12 engine core for a Neon Pong game at `D:\Test\repos\PongGame\src\`.
2026-05-12T00:38:56.764Z [DEBUG] General-purpose agent: agentModelSetting="capi:Qwen_Qwen3.6-35B-A3B-Q4_K_M.gguf", currentModel="Qwen_Qwen3.6-35B-A3B-Q4_K_M.gguf", modelOverride="(none)", resolvedModel="Qwen_Qwen3.6-35B-A3B-Q4_K_M.gguf"
2026-05-12T00:38:56.764Z [INFO] General-purpose agent invoked with prompt: Create the complete Pong game logic files at `D:\Test\repos\PongGame\src\`.
2026-05-12T00:38:56.839Z [ERROR] Command failed with exit code 128: git rev-parse HEAD
2026-05-12T00:38:56.839Z [INFO] --- End of group ---
```

No JavaScript exception or handled stack trace appeared in the process log before exit. The Windows crash event followed immediately afterward.

## Notes

Before the crash, the local model had produced some malformed Windows path/tool calls in earlier turns, but this crash appears to have happened specifically during parallel subagent startup under custom-provider mode. A failed `git rev-parse HEAD` in a non-git directory should not crash the CLI.

I can provide the full process log privately if helpful, but I am not attaching it to this public issue because it may include prompts and local paths.