Add daily auto-update workflow with shared build script

.gitea/scripts/build-and-cache.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Configure Attic cache
+attic login local "$ATTIC_ENDPOINT" "$ATTIC_TOKEN"
+attic use local:nixos
+
+# Check flake
+nix flake check --all-systems --print-build-logs --log-format raw --show-trace
+
+# Build all systems
+nix eval .#nixosConfigurations --apply 'cs: builtins.attrNames cs' --json \
+  | jq -r '.[]' \
+  | xargs -I{} nix build ".#nixosConfigurations.{}.config.system.build.toplevel" \
+      --no-link --print-build-logs --log-format raw
+
+# Push to cache (only locally-built paths >= 0.5MB)
+toplevels=$(nix eval .#nixosConfigurations \
+  --apply 'cs: map (n: "${cs.${n}.config.system.build.toplevel}") (builtins.attrNames cs)' \
+  --json | jq -r '.[]')
+echo "Found $(echo "$toplevels" | wc -l) system toplevels"
+paths=$(echo "$toplevels" \
+  | xargs nix path-info -r --json \
+  | jq -r '[to_entries[] | select(
+      (.value.signatures | all(startswith("cache.nixos.org") | not))
+      and .value.narSize >= 524288
+    ) | .key] | unique[]')
+echo "Pushing $(echo "$paths" | wc -l) unique paths to cache"
+echo "$paths" | xargs attic push local:nixos