Add .gitea/workflows/build.yaml: a `checks` job running a static matrix of all eight flake checks (clippy, fmt, toml-fmt, audit, deny, docs, nextest, llvm-cov) and a `package` job building `.#kobors` and `.#kobors-web-ui`, both on the Nix-enabled `nix` runner. The matrix is static because Gitea Actions cannot expand a dynamic matrix from a prior job's outputs (gitea#25179). Remove .github/workflows/{build,docs}.yaml: GitHub-template leftovers that referenced nonexistent `hello-*` check attrs and GitHub-only services (magic-nix-cache, codecov, Pages). Gitea scans .github/workflows too, so they would fail on every push. The repo's only remote is Gitea.