ci: isolate PR checks from self-hosted runner

[jmcte]

Motivation

• A security scan flagged that the pull_request workflow checked out PR-controlled code and executed repository scripts on a self-hosted macOS runner, enabling arbitrary code execution from untrusted PRs.
• The intent is to prevent untrusted PR content from running on persistent self-hosted infrastructure that may contain secrets or sensitive local state.
• The change also ensures CI policy checks catch regressions that would reintroduce the unsafe routing.

Description

• Split the original CI job into two jobs and added a PR-only job pr-ci-gate with if: github.event_name == 'pull_request' that uses a GitHub-hosted runner via runs-on: - macos-15 and runs the existing bash scripts/ci/run-fast-checks.sh steps.
• Added a trusted-ci-gate job with if: github.event_name != 'pull_request' that retains the self-hosted runner labels (self-hosted, macOS, ARM64, xcode) for trusted push/workflow_dispatch events.
• Updated scripts/ci/check-ci-policy.sh to require the PR workflow include the - macos-15 line so local/CI validation will detect regressions.
• Preserved existing steps such as actions/checkout@v4 and xcodebuild -version in both job paths.

Testing

• Ran bash scripts/ci/check-ci-policy.sh and it completed successfully.
• Ran bash scripts/ci/check-shell-syntax.sh and it completed successfully.
• Ran swift test and the test suite completed successfully.

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[athena-omt]

APPROVE

The PR does what it says: pull requests now run on a GitHub-hosted macos-15 runner, while trusted push/workflow_dispatch paths stay on the self-hosted macOS ARM64 runner. The policy check was tightened to catch regressions in the PR workflow routing.

Evidence checked:

• .github/workflows/pr-fast-ci.yml
• scripts/ci/check-ci-policy.sh

Validation:

• bash scripts/ci/check-ci-policy.sh
• bash scripts/ci/check-shell-syntax.sh

Residual risk is low; I did not find any remaining path where pull_request events can hit the self-hosted runner in this workflow.