perf(ci): bump default runner (4→8/16-core) — follow-up to #363

[scottschreckengaust]

Parent: #363. Spun from #363's DoD by the orchestrator; needs maintainer approved (ADR-003).

Goal

Raise the default runner size via vars.DEFAULT_RUNNER_LABEL (mechanism already exists in build.yml). jest maxWorkers + CDK synth are CPU-bound and both benefit.

Scope / caveats

• Weigh recurring per-run cost vs wall-time saved.
• A/B against the sharding approach (#shard-issue) — they may be substitutes, not both.

Refs #363