A distinct `flake` job runs `nix flake check` on push and pull request,
catching flake rot — most concretely a build-relevant file left out of
package.nix's source allowlist — at the commit that causes it rather than
weeks later at the maintainer's next system rebuild.
The job is non-gating by two mechanisms: no `needs` edge, so it neither
waits on the test job nor is waited on, and `continue-on-error` so a red
build does not fail the run. That flag sits on the steps rather than the
job because Gitea's `act` fork declares it on its Step struct only and
silently ignores the job-level key; recorded as a Gotcha.
