[Repo Assist] fix(uninstall): clean up empty wsl\ parent dir after VHD removal

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Summary

After the WSL uninstall flow removes the distro-specific VHD directory (%LOCALAPPDATA%\OpenClawTray\wsl\OpenClawGateway\), the parent wsl\ directory was left as an empty orphan. This prevented the MSIX uninstaller from removing %LOCALAPPDATA%\OpenClawTray\ cleanly.

Closes #467

Root Cause

Step 5a in LocalGatewayUninstall.cs calls Directory.Delete(vhdDir, recursive: true) on the distro-specific subdirectory but does not clean up the wsl\ parent. After all 13 steps complete, the empty wsl\ directory holds %LOCALAPPDATA%\OpenClawTray\ open, so it survives MSIX uninstall.

Fix

• Step 5b added to LocalGatewayUninstall.cs: after VHD parent dir cleanup, delete the wsl\ parent directory if it is empty. Non-empty wsl\ dirs (e.g. other distros still present) are preserved safely.
• WslParentDirAbsent postcondition added; included in AllRequiredPostconditionsMet and AppendPostconditionErrors.
• validate-wsl-gateway-uninstall.ps1 updated: added $wslParentDirPath, wsl_parent_dir_absent to postconditions and required-checks list, and wsl_parent_dir_exists in pre-state snapshot.
• 3 unit tests added covering empty→deleted, non-empty→skipped, and already-absent→skipped.

Trade-offs

• The wsl\ cleanup is guarded with EnumerateFileSystemEntries(...).Any() — it will not delete the directory if any other files or subdirectories remain, so this is safe for machines that have other WSL distros under %LOCALAPPDATA%\OpenClawTray\wsl\.
• The %LOCALAPPDATA%\OpenClawTray\ root directory itself is left for the MSIX uninstaller to clean up once empty, which is consistent with the existing pattern.

Test Status

dotnet build tests/OpenClaw.Tray.Tests/ — Build succeeded, 0 warnings
dotnet test tests/OpenClaw.Tray.Tests/ -- no-restore
  Failed: 9 (all pre-existing Linux-environment failures)
  Passed: 1065
  Skipped: 51 (includes 3 new WslParentDirCleanup_* tests, skipped on Linux per [WindowsFact])

dotnet test tests/OpenClaw.Shared.Tests/ -- no-restore
  Failed: 7 (all pre-existing)
  Passed: 1796

All failures are pre-existing. No regressions introduced.

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