ci: add tzdata update workflow

Author: github-actions[bot]

.github/workflows/tzdata-update.yml

@@ -0,0 +1,51 @@
+name: tzdata update
+on:
+  schedule:
+    - cron: "0 23 * * *"
+  workflow_dispatch:
+
+permissions:
+  contents: write
+
+jobs:
+  update:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repository code
+        uses: actions/checkout@v4
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+      - name: Fetch latest tzcode version
+        run: |
+          set -euo pipefail
+          workdir=$(mktemp -d)
+          wget -O "$workdir/tzcode-latest.tar.gz" https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
+          tar -xzf "$workdir/tzcode-latest.tar.gz" -C "$workdir"
+
+          version_file=$(find "$workdir" -maxdepth 2 -type f -name version | head -n 1)
+          if [ -z "$version_file" ]; then
+            echo "version file not found in tzcode-latest.tar.gz" >&2
+            exit 1
+          fi
+
+          latest_version=$(tr -d ' \n' < "$version_file")
+          current_version=$(grep -E 'tzVersion' scripts/tzgen/main.go | sed -E 's/.*"([^"]+)".*/\1/' | head -n 1)
+
+          echo "current=$current_version latest=$latest_version"
+          if [ "$latest_version" = "$current_version" ]; then
+            echo "tzdata is already up to date"
+            exit 0
+          fi
+
+          sed -i.bak -E 's/tzVersion = ".*"/tzVersion = "'"$latest_version"'"/' scripts/tzgen/main.go
+          rm -f scripts/tzgen/main.go.bak
+          gofmt -w scripts/tzgen/main.go
+          go run scripts/tzgen/main.go
+
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+          git add scripts/tzgen/main.go tz
+          git commit -m "Update tzdata to ${latest_version}"
+          git push