Fix run-smoke.yml overlapping-runs failure

[saqibmanan]

Summary

• Root cause of today's "Smoke Tests" failure: PRs e2e round 2: re-verify, fix locator drift, add evaluator-review coverage #7 and Resume e2e-tests in CI #8 merged 37 seconds apart, each independently triggering a push-to-main run of run-smoke.yml against the shared dev backend. This workflow had no concurrency: guard (unlike ci.yml, which got one 2026-08-12 for the identical reason).
• The second run overlapped the first, hit a hard 120s pytest-timeout kill on a test that hangs under concurrent backend load, and correctly failed since no report.json was produced — but for an infra reason, not a code defect.
• Fix: same concurrency block ci.yml already uses, adapted for this workflow's triggers (push/workflow_dispatch/workflow_call — no pull_request, so grouped by github.ref alone).

Test plan

• actionlint clean
• Confirm two close-together pushes to main no longer produce an overlapping smoke run (second cancels the first instead of contending for the backend)