launch_engine ACCESS_VIOLATION (0xC0000005) on Windows after auto-update replaces binary

### Describe the bug

After `copilot.exe` is auto-updated (WinGet package replaced in-place), the **next launch** immediately crashes with `exit code -1073741819` (`0xC0000005` — ACCESS_VIOLATION) at the `launch_engine` step. No session starts — the crash happens before any interaction.

A retry shortly after succeeds with the same binary, suggesting a **race condition** between the auto-updater replacing the binary and the launch attempt.

### Affected version

GitHub Copilot CLI 1.0.26-0

### Environment

- **OS:** Windows 11 Insider (10.0.26200), x64
- **Install method:** WinGet (`GitHub.Copilot_Microsoft.Winget.Source`)
- **Terminal:** Windows Terminal
- **Copilot plan:** Enterprise

### Evidence of auto-update as trigger

```
copilot.exe — 2026-04-14 13:52:23 (SHA256: 73A491F4C31AEF24F7E2DA68D9227A93B201C3B1167BE42810B37803FEE9717D) <- new
copilot.exe.old — 2026-04-12 12:11:50 (SHA256: 10A6B746EE7A1A216084F10AE316BCE2057BE4474B95FF133BDF2DA3C9391B30) <- previous
```

The crash occurred at 15:08 (local), ~75 minutes after the binary was replaced. The `.old` backup confirms the auto-updater ran. A retry at ~15:19 succeeded with the identical new binary.

### Crash output

```
🤖 Agency 2026.4.11.1
📦 Resolving Copilot CLI...
🧠 Copilot CLI at ...\GitHub.Copilot_Microsoft.Winget.Source_8wekyb3d8bbwe\copilot.exe
✅ Copilot CLI resolved (in 143 ms)
🔌 Loaded 9 MCP server(s) from ~/.copilot/mcp-config.json
Error: Agent execution failed: launch_engine - copilot.exe exited with non-zero status (exit code: -1073741819)
```

### Hypothesis

The auto-updater writes `copilot.exe` while the old file may still be locked (Windows Defender scan, file system cache, or another copilot process). The replacement completes (the file exists and `--version` works), but the binary may be in an inconsistent state for engine launch until the OS fully releases handles/caches. This creates a transient window where launching immediately after update fails with ACCESS_VIOLATION.

### What I've ruled out

- **Binary corruption:** `copilot.exe --version` returns `1.0.26-0` successfully — the binary itself is valid
- **pkg cache:** No `~/.copilot/pkg/universal/` directory exists (clean state)
- **PATH issues:** Single PATH entry pointing to correct binary
- **session-store.db:** 7.1 MB, not corrupt (valid SQLite header)
- **Persistent crash:** Retry with identical binary succeeds — crash is transient

### Expected behavior

1. The auto-updater should ensure atomic binary replacement (write to temp -> rename, not in-place overwrite)
2. If launch fails immediately after update, the CLI should detect this and suggest retrying or rolling back to `.old`
3. Ideally: a short validation check after update before handing off to the new binary

### Related

- #2387 — Intermittent ACCESS_VIOLATION during mid-session (same exit code, different trigger — possibly same underlying native crash, different entry point)