Add missing shell directive to PowerShell build steps

[Copilot]

Addresses feedback from #23 regarding missing shell: pwsh directives in build steps using PowerShell syntax.

Changes

• Added explicit shell: pwsh to "Build Solution" step (line 87)
• Added explicit shell: pwsh to "Build Debug Configuration" step (line 193)

Both steps use PowerShell-specific syntax (Join-Path, $variable, backtick continuation) but were relying on implicit shell selection, which could cause inconsistent behavior across different runner configurations.

- name: Build Debug Configuration
  working-directory: ${{ github.workspace }}
  shell: pwsh  # ← Added
  run: |
    $outDir = Join-Path "${{ env.BUILD_OUTPUT_ROOT }}" "${{ matrix.platform }}\Debug\Binary\"
    msbuild /m `
      /p:Configuration=Debug `
      ...

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