Adds `.forgejo/workflows/build-iso.yml` that runs `./iso/build.sh` and
uploads the resulting ISO as a `furtka-iso` artifact (retained 14 days).
Triggers on `push: branches: [main]` and `workflow_dispatch` only —
feature branches don't pay the 15-20 min build cost. `concurrency`
cancels older runs of the same ref so only the most recent push
produces an artifact.
This is what Robert asked for: push change → download ISO from the
Forgejo run → test without needing a laptop to build.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
