[spacedevin]

Motivation

August 2026 billed ~23,000 Linux minutes on this repo. ~88% of that is the verify matrix (90 jobs, each billed rounded up to whole minutes, ~250 billed minutes per run). There were 59 PR runs and 44 main runs. ci.yml had no concurrency block, so every push to a PR branch left the previous run grinding through the full matrix for a commit that had already been replaced.

Change

Add a top-level concurrency block to ci.yml and screenshot.yml:

concurrency:
  group: ci-${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
  cancel-in-progress: ${{ github.event_name == 'pull_request' }}

• PR runs are grouped by PR number, so a new push cancels the superseded run.
• Pushes to main are grouped by ref but cancel-in-progress is false for them: the main run cuts the prerelease and is the run chuggie.dev reads example-roms-verify-<id> from via nightly.link, so a superseded main run still has to finish.
• The group is prefixed with the workflow name so the two workflows never cancel each other.

Nothing else changes. Both files parse (actionlint, ruby YAML.load_file); the one shellcheck note actionlint reports on ci.yml (SC2155 in the gba-shot build step) is pre-existing on main.