feat: migrate CI and releases from GitHub Actions to Gitea Actions

Retarget GoReleaser to publish to Gitea (gitea_urls, release.gitea,
changelog.use: gitea). Add Gitea Actions workflows for build/test,
release, and benchmarks — adapted from GitHub equivalents with macOS
jobs and SLSA provenance dropped. Old .github/workflows/ kept in place.

.gitea/workflows/main.yml

@@ -0,0 +1,115 @@
+name: Build and test
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+          cache: true
+
+      - name: Download dependencies
+        run: go mod download
+
+      - name: Build
+        run: make build-ci
+
+  lint:
+    name: Lint
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+          cache: true
+
+      - name: Download dependencies
+        run: go mod download
+
+      - name: Lint
+        uses: golangci/golangci-lint-action@v6
+        with:
+          install-mode: goinstall
+          version: v1.64.8
+
+  test-linux:
+    name: Test (Linux)
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+          cache: true
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Set up Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+
+      - name: Download dependencies
+        run: go mod download
+
+      - name: Install Linux sandbox dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y \
+            bubblewrap \
+            socat \
+            uidmap \
+            curl \
+            netcat-openbsd \
+            ripgrep
+          # Configure subuid/subgid for the runner user (required for unprivileged user namespaces)
+          echo "$(whoami):100000:65536" | sudo tee -a /etc/subuid
+          echo "$(whoami):100000:65536" | sudo tee -a /etc/subgid
+          # Make bwrap setuid so it can create namespaces as non-root user
+          sudo chmod u+s $(which bwrap)
+
+      - name: Verify sandbox dependencies
+        run: |
+          echo "=== Checking sandbox dependencies ==="
+          bwrap --version
+          socat -V | head -1
+          echo "User namespaces enabled: $(cat /proc/sys/kernel/unprivileged_userns_clone 2>/dev/null || echo 'check not available')"
+          echo "Kernel version: $(uname -r)"
+          echo "uidmap installed: $(which newuidmap 2>/dev/null && echo yes || echo no)"
+          echo "subuid configured: $(grep $(whoami) /etc/subuid 2>/dev/null || echo 'not configured')"
+          echo "bwrap setuid: $(ls -la $(which bwrap) | grep -q '^-rws' && echo yes || echo no)"
+          echo "=== Testing bwrap basic functionality ==="
+          bwrap --ro-bind / / -- /bin/echo "bwrap works!"
+          echo "=== Testing bwrap with user namespace ==="
+          bwrap --ro-bind / / --unshare-user --uid 0 --gid 0 -- /bin/echo "bwrap user namespace works!"
+
+      - name: Run unit and integration tests
+        run: make test-ci
+
+      - name: Build binary for smoke tests
+        run: make build-ci
+
+      - name: Run smoke tests
+        run: GREYWALL_TEST_NETWORK=1 ./scripts/smoke_test.sh ./greywall