ci: add build and test workflow for Linux and Windows

Runs build, doctor, the platform-package tests, the managed-manifest
tests, the build-validation tests, and an install dry run on both
ubuntu-latest and windows-latest for every push to main and every pull
request.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -0,0 +1,54 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  linux:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build
+        run: ./scripts/build.sh
+      - name: Doctor
+        run: ./scripts/doctor.sh
+      - name: Platform package tests
+        run: ./scripts/test-platform-packages.sh
+      - name: Managed manifest tests
+        run: ./scripts/test-managed-install.sh
+      - name: Build validation tests
+        run: ./scripts/test-build-validation.sh
+      - name: Install dry run (isolated HOME)
+        run: |
+          export HOME="$RUNNER_TEMP/isolated-home"
+          mkdir -p "$HOME"
+          ./scripts/install.sh --dry-run --client both --platform linux
+
+  windows:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build
+        shell: powershell
+        run: .\scripts\build.ps1
+      - name: Doctor
+        shell: powershell
+        run: .\scripts\doctor.ps1
+      - name: Platform package tests
+        shell: powershell
+        run: .\scripts\test-platform-packages.ps1
+      - name: Managed manifest tests
+        shell: powershell
+        run: .\scripts\test-managed-install.ps1
+      - name: Build validation tests
+        shell: powershell
+        run: .\scripts\test-build-validation.ps1
+      - name: Install dry run
+        shell: powershell
+        # -DryRun performs no writes, so it is safe to run against the runner's
+        # real profile path. [Environment]::GetFolderPath('UserProfile') reads
+        # from the Windows profile API on PowerShell 5.1, not $env:USERPROFILE,
+        # so overriding that variable would not relocate it anyway.
+        run: .\scripts\install.ps1 -DryRun -Client Both -Platform Windows