Convert Jenkins CI to Gitea Actions

Replace the Jenkinsfile with .gitea/workflows/ci.yml. The CI image is
built from the Dockerfile and pushed to the Gitea container registry,
rebuilt only when the Dockerfile or pre-commit config changes. The
aarch64 release build now runs natively on an arm64 runner instead of
cross-compiling, so the cross toolchain and qemu are dropped from the
image. Artifacts still go to MinIO (via mc, skipped gracefully if
credentials are not configured); ccache uses actions/cache instead of a
host volume.

Dockerfile

@@ -8,13 +8,11 @@ RUN chmod -R 777 /tmp
 RUN apt-get update
 RUN apt-get upgrade -y
 RUN TZ=America/Los_Angeles DEBIAN_FRONTEND=noninteractive apt-get install -y \
-    binutils-aarch64-linux-gnu \
     build-essential \
     ccache \
     cmake \
     curl \
     devscripts \
-    g++-aarch64-linux-gnu \
     gcovr \
     git \
     gnupg \
@@ -24,13 +22,16 @@ RUN TZ=America/Los_Angeles DEBIAN_FRONTEND=noninteractive apt-get install -y \
     ninja-build \
     pre-commit \
     python3-requests \
-    qemu-user \
     rpm \
     software-properties-common \
     texlive-full \
     wget \
     zstd
 
+# MinIO client, for uploading build artifacts
+RUN curl -Ls "https://dl.min.io/client/mc/release/linux-$(dpkg --print-architecture)/mc" \
+    -o /usr/local/bin/mc && chmod +x /usr/local/bin/mc
+
 # Install recent valgrind from source
 RUN curl -Ls https://sourceware.org/pub/valgrind/valgrind-3.22.0.tar.bz2 -o valgrind.tar.bz2 && \
     echo "c811db5add2c5f729944caf47c4e7a65dcaabb9461e472b578765dd7bf6d2d4c  valgrind.tar.bz2" > valgrind-sha.txt && \