fix(ps): drop ssh probe + correct exit-code check in Invoke-GhRepoClone

[szymonos]

Summary

Two follow-up fixes for code-review feedback on the SSH-first path introduced in #246 (`modules/InstallUtils/Functions/git.ps1`).

1. Drop the `ssh -T git@github.com` probe

The probe ran on every `Invoke-GhRepoClone` call to decide between SSH and HTTPS. Three problems:

• Network round-trip per call. Every PowerShell module install during `linux_setup.sh` paid this cost.
• Can block. Host-key prompt (no entry in `~/.ssh/known_hosts`), slow DNS, firewall holes, encrypted-key passphrase prompts.
• Redundant. The catch block already retries SSH failures over HTTPS, so the probe was defensive over-engineering.

Protocol selection reverts to deriving from the parent repo's remote URL via `getOrigin` (the original pre-#246 behavior). SSH-configured parent repos still trigger SSH-first cloning, and the HTTPS retry remains as fallback.

The `Get-Command ssh` guard added in #247 (for Windows hosts without OpenSSH) is now obsolete — there's no bare `ssh` invocation to guard.

2. Replace pipeline `$?` check with `$LASTEXITCODE`

```powershell
git clone $cloneUrl "$destPath" --quiet 2>&1 | ForEach-Object { $cloneErr += "$_`n" }
if (-not $?) { ... } # WRONG: $? reflects ForEach-Object (always true)
```

The pipeline made `$?` reflect the tail (`ForEach-Object`, which always succeeds), so failed clones were treated as successful: the HTTPS fallback never ran and callers received `status=1` (cloned) when nothing was actually fetched. Now `$LASTEXITCODE` is consulted directly and captured stdout+stderr is rendered via `Out-String` for the warning text.

```powershell
$cloneOut = git clone $cloneUrl "$destPath" --quiet 2>&1
$cloneOk = $LASTEXITCODE -eq 0
if (-not $cloneOk -and $gitProtocol -eq 'git@github.com:') {
# ... HTTPS retry ...
}
```

Test plan

• `pwsh` parse-check on `modules/InstallUtils/Functions/git.ps1`
• `make lint` clean (gremlins / shellcheck / shfmt / pester / bats all pass)
• Manual: run `linux_setup.sh` on a fresh distro with `-Scope shell` to validate the ps-modules clone still works (HTTPS path)
• Manual: same on a distro with SSH key configured to validate the SSH path still triggers when parent repo origin is SSH-form

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