fix(discovery): spawn PowerShell install commands natively on Windows

[wpfleger96]

Problem

On Windows, install_shell_command wraps every install command in Git Bash -l -c, including the Windows-specific powershell.exe … irm https://chatgpt.com/codex/install.ps1 | iex command. A Git Bash login shell prepends its POSIX dirs (C:\Program Files\Git\usr\bin) to PATH, so when Codex's install.ps1 shells out to bare tar -xzf C:\…, it resolves Git's GNU tar (/usr/bin/tar) instead of Windows bundled bsdtar. GNU tar parses C: as a remote host:

tar (child): Cannot connect to C: resolve failed
gzip: stdin: unexpected end of file
/usr/bin/tar: Child returned status 128
/usr/bin/tar: Error is not recoverable: exiting now
Downloaded Codex package archive did not contain the expected package layout.

Claude's installer doesn't hit the same failure because it doesn't shell out to tar.

Solution

On Windows, detect powershell.exe install commands and spawn them natively (Command::new("powershell.exe")) instead of routing them through Git Bash. The discriminator is a case-insensitive prefix check on the first whitespace-delimited token — minimal and precise.

The native spawn preserves everything install_shell_command provides that applies:

• NPM_CONFIG_* / COREPACK env strip + managed npm prefix env
• PATH composed from managed Buzz dirs + inherited process PATH (no POSIX login-shell dirs)
• CREATE_NO_WINDOW so no console flash
• stdin null, piped-drain in run_install_command
• The retry/backoff/annotate logic is fully shared

The -Command body is split correctly at the boundary (case-insensitive) and passed as a single argument to preserve pipes and spaces inside the installer script call.

Non-PowerShell commands (e.g. npm install -g … adapter steps) continue through the existing Git Bash path unchanged.

Tests

6 unit tests:

1. is_powershell_command detection (positive + negative)
2. Routing: PowerShell → native spawn on Windows, non-PowerShell → Git Bash
3. Unix: non-Windows path returns the shell command unchanged (compile-time cfg)
4. -Command body preservation (no bash args in native spawn; body is single arg)

Full just desktop-tauri-test suite: 1627/1627 passing. Windows CI will validate end-to-end.