ci: add GitHub Actions workflow for Pester tests

Runs on ubuntu, windows, and macos with PowerShell 7.
Uploads NUnit XML test results as artifacts.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: ericchansen

.github/workflows/ci.yml

@@ -0,0 +1,42 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    name: Pester Tests — ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Pester
+        shell: pwsh
+        run: Install-Module Pester -Force -SkipPublisherCheck -Scope CurrentUser -MinimumVersion 5.0
+
+      - name: Run tests
+        shell: pwsh
+        run: |
+          Import-Module Pester -MinimumVersion 5.0
+          $cfg = New-PesterConfiguration
+          $cfg.Run.Path = './tests'
+          $cfg.Output.Verbosity = 'Detailed'
+          $cfg.TestResult.Enabled = $true
+          $cfg.TestResult.OutputFormat = 'NUnitXml'
+          $cfg.TestResult.OutputPath = './test-results.xml'
+          Invoke-Pester -Configuration $cfg
+
+      - name: Upload test results
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: test-results-${{ matrix.os }}
+          path: test-results.xml