Extract checkout, nix, signing and the Gradle run into a reusable build
workflow used by CI, the next build and the release build job. The build
job holds only the signing secrets; the publish job is the only one with
a write token and the Play credential. The Play service account is now
written to a 0600 file and passed via --json_key instead of on argv.
The v4 pair targets Node 20, which GitHub now forces onto Node 24 (Node 20 is
deprecated). The Forgejo runner is v13, which runs Node 24, so both copies move
together. Pin the GitHub runner to ubuntu-26.04: ubuntu-latest migrates to it
over Oct 19 - Nov 19, 2026, and Nix supplies the toolchain either way.
Add .github/workflows/ci.yml, a copy of the Forgejo CI workflow adapted to
GitHub-hosted runners (ubuntu-latest + cachix/install-nix-action), so pull
requests on the GitHub mirror get the lint, unit-test and integration checks.
The GitHub copy uses a 90 minute timeout so a cold run can finish and warm the
caches, caps the Nix cache so it cannot evict the Gradle/music caches from
GitHub's 10 GB per-repo budget, and pins the job token to contents:read.
austinried