ci: migrate PR checks to GitHub Actions

[Copilot]

• Create GitHub Actions PR workflow (.github/workflows/pr.yml)
• Add catch-all PR job that depends on all other checks
• Fix Windows SDK version mismatch on windows-latest runner (use 10.0.26100.0, the only SDK on Windows Server 2025)
• Update setup-toolchain to @5.0.14
• Delete old Azure Pipelines PR checks (.ado/azure-pipelines.yml)
• Run Android PR job on ubuntu-latest instead of macos-latest
• Use macos-latest for macOS and iOS PR jobs

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[changeset-bot]

⚠️ No Changeset found

Latest commit: 0acc1a6

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[Saadnajmi]

@copilot can we setup our toolchain in the PR using the RNTA action, like so?

      - name: Set up toolchain
        uses: microsoft/react-native-test-app/.github/actions/setup-toolchain@5.0.14

[Copilot]

@copilot can we setup our toolchain in the PR using the RNTA action, like so?

      - name: Set up toolchain

...

Done — updated all 7 references from @4.4.5 to @5.0.14 in 30cf1d4.

[Saadnajmi]

@copilot the windows PR check still consistently fails

[Copilot]

@copilot the windows PR check still consistently fails

The windows-latest runner (Windows Server 2025) only has SDK 10.0.26100.0 installed — both 10.0.19041.0 and 10.0.22621.0 are absent. Updated the --msbuildprops to target 10.0.26100.0 in 3e2a40c.

[Saadnajmi]

@copilot lets' delete the old Azure Pipelines PR checks

[Copilot]

@copilot lets' delete the old Azure Pipelines PR checks

Deleted .ado/azure-pipelines.yml in 3f40495. The publish pipeline (.ado/azure-pipelines.publish.yml) and shared resources (scripts, templates, variables, config) are kept since they're still used by the publish pipeline and the new GHA workflow.