[codex] speed up switchyard ci

[fdatoo]

Summary

• Split the long Switchyard platform build/test workflow into separate Ubuntu build/unit, Ubuntu race, Ubuntu integration, and macOS build/unit jobs.
• Keep macOS race and integration coverage on non-PR runs while avoiding that cost on every PR.
• Fix workflow linting by defining the exception field used by coverage matrix notices.

Impact

The PR critical path should no longer be one serial per-OS job that runs build, unit, race, and integration back-to-back. The heaviest checks can run concurrently, with macOS limited to platform compatibility on PRs.

Validation

• actionlint .github/workflows/ci.yml
• go build -o dist/switchyard ./cmd/switchyard && go test -tags=integration ./... with generated web assets temporarily removed
• go test -race ./internal/interestingness ./internal/eventstore ./internal/registry ./internal/carport
• pre-push task test, task test:race, task test:integration