ci: downsize pg/integration/ai test tiers from 4vcpu to 2vcpu

[pyramation]

Summary

Drops the pg-tests, integration-tests, and ai-tests tiers in run-tests.yaml from blacksmith-4vcpu-ubuntu-2404 to blacksmith-2vcpu-ubuntu-2404. On a recent main run these ~30 matrix jobs each spend only 1–21s in the actual Test step — the ~45–60s of fixed setup (artifact download, pnpm install, Postgres tune/seed, fixtures:install) dominates. They are not CPU-bound, so 4vCPU is wasted spend; 2vCPU should roughly halve their cost with negligible wall-clock change.

Also pins NODE_OPTIONS=--max-old-space-size=4096 on those tiers, since on smaller runners Node's memory-derived default heap can land near ~2GB and OOM Jest (observed in constructive-db's equivalent experiment).

Unchanged: build and unit-tests stay on ubuntu-latest.

Follow-up worth considering (not in this PR): the many tiny pg/integration jobs each re-pay ~45s of fixed overhead; batching them (like the unit-tests tier already does) would cut cost further than runner sizing alone.

Link to Devin session: https://app.devin.ai/sessions/180599d6e0c1419994599efb1eabd9b5
Requested by: @pyramation

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