Cancel superseded CI runs with a concurrency group

[Nomadcxx]

Problem

None of the workflows in this repo set concurrency, so every push starts its own run: a burst of pushes to a branch or PR queues a full build per push instead of cancelling the superseded one. Each of those runs can fail independently and each failure produces a notification.

Approach

Add a concurrency group keyed on workflow + ref:

• workflow files that run on branch/PR pushes: cancel-in-progress: true — a new push cancels the in-flight run for that branch.
• release.yml: cancel-in-progress: false — a release must not be cancelled mid-flight, so superseded runs queue instead of killing the running one.

No trigger, job, or step logic is changed. The diff is insert-only.

Verification

• Each modified file parses as YAML and the parsed concurrency.cancel-in-progress value was read back to confirm it is true for CI-style workflows and false for the release workflow.
• git diff --stat shows insert-only changes.