Adds `services.hyperhive.ci` NixOS module that spins up a `hive-ci`
nixos-container running `gitea-actions-runner` against the hive-forge
Forgejo instance. Off by default; opt in with `ci.enable = true` after
generating a runner registration token in Forgejo.
Also adds `.forgejo/workflows/ci.yml` with four jobs: nix flake check,
formatting (nix fmt + cargo fmt), cargo test, and cargo clippy. Jobs
target the `hive-ci` runner label.
Container design mirrors hive-forge (shared host netns, non-ephemeral
state, loopback reach to forge). sandbox-fallback = true since nspawn
containers can't create user-namespaces for nix sandbox.
Closes#175.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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