[Bugs] On Windows, agent-merge skill scripts run under WSL bash and fail (bundled gh/jq not on WSL PATH)

### Short summary

On Windows, the agent-merge skill runs its helper script with bare `bash` (e.g. `bash check-merge-conditions.sh `). On a standard Windows dev box that has WSL installed, `bash` resolves to `C:\Windows\System32\bash.exe` (the WSL launcher), so the script executes **inside the WSL distro**, where the Copilot-bundled `gh` and the user's `jq` are not on PATH. The script fails immediately with `No such file or directory` / exit 127, so agent-merge cannot evaluate merge conditions.

### Affected version or release

- GitHub Copilot app: 0.2.27
- Copilot CLI: 1.0.60
- OS: Windows 11 Enterprise (10.0.26200)
- WSL: installed (default distro Ubuntu-22.04)
- Git for Windows: 2.53.0.windows.3 (Git Bash present at `C:\Program Files\Git\bin\bash.exe` but NOT on PATH, which is the Git for Windows default)

### What happened?

When agent-merge follows the skill instructions and runs `bash check-merge-conditions.sh ...`:

```
/bin/bash: C:Users...app-skills\agent-merge\check-merge-conditions.sh: No such file or directory
# exit 127
```

The script never runs because:
1. `bash` resolves to the WSL launcher (`System32\bash.exe`), not Git Bash.
2. Inside WSL, the Copilot-bundled `gh` (`%LOCALAPPDATA%\copilot-desktop-gh-*\gh`) and the user's `jq` (mise shim) are not available, so even when the path is corrected the script's `gh`/`jq` calls fail.

Confirmed root cause on this machine: the only `bash.exe` entries on PATH are the two WSL launchers (`System32`, `WindowsApps`); `C:\Program Files\Git\bin` is intentionally not on PATH (Git for Windows default). So bare `bash` -> WSL is the out-of-the-box outcome.

### Why this is not environment-specific

This reproduces for the common configuration class "Git for Windows (default install) + WSL installed", not a one-off misconfiguration:
- Git for Windows deliberately keeps `bash` off PATH to avoid shadowing system tools.
- WSL places `bash.exe` in `System32`.
- Therefore bare `bash` -> WSL is the default behavior for a large population of Windows dev machines, and the skill always invokes bare `bash`.

It happens on every agent-merge run/session on such a machine.

### Workaround

Invoking the script through Git Bash explicitly works, because Git Bash inherits the Windows PATH and can see the bundled `gh` and the `jq` shim:

```
& "C:\Program Files\Git\bin\bash.exe" -lc "/c/Users/.../agent-merge/check-merge-conditions.sh "
```

(With Git Bash the WSL/PATH problem is gone; a separate `Failed to fetch requested reviewers` / exit 2 can still occur from the script's reviewer fetch, but that is a different layer.)

### Suggested fixes

1. When running skill `.sh` scripts on Windows, prefer Git Bash (`C:\Program Files\Git\bin\bash.exe`) over a bare `bash` that resolves to WSL.
2. Or ensure the interpreter used for skill scripts can see the Copilot-bundled `gh` and `jq`.
3. Or provide a PowerShell-native execution path / fallback for these skills on Windows.

### Related

- github/copilot-cli#508 (Support Git Bash)
- github/copilot-cli#2981 (Copilot CLI uses Unix-only commands on Windows)