Incremental package builds and releases on v* tags (0.8.0)

- build-packages.sh rebuilds a package only when its srcpkgs/ directory
  changed, its binary package is missing, or a dry-run install finds a
  shared library Void no longer provides; DASOS_REBUILD=1 rebuilds all.
  An unchanged `make packages` takes 20 s instead of 17 min on the CI host
- ci/release.sh publishes both ISOs and SHA256SUMS as a Gitea release
  when the workflow runs for a v* tag matching VERSION
- README and TODO updated

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ci/release.sh

@@ -0,0 +1,41 @@
+#!/bin/sh
+# Publishes the ISO images of the current version as a Gitea release:
+#   release.sh <tag> <iso dir>
+# Needs GITEA_TOKEN, GITEA_SERVER_URL and GITEA_REPOSITORY (owner/name); in CI
+# the job provides them. Running it again for the same tag replaces the files.
+set -eu
+
+tag=$1
+dir=$2
+version=$(cat VERSION)
+[ "$tag" = "v$version" ] || { echo "release: tag $tag does not match VERSION $version" >&2; exit 1; }
+
+api=$GITEA_SERVER_URL/api/v1/repos/$GITEA_REPOSITORY
+auth="Authorization: token $GITEA_TOKEN"
+json() { python3 -c "import json,sys; d=json.load(sys.stdin); $1"; }
+
+isos="dasos-$version-x86_64.iso dasos-server-$version-x86_64.iso"
+cd "$dir"
+sha256sum $isos >SHA256SUMS
+
+if ! id=$(curl -fsS -H "$auth" "$api/releases/tags/$tag" 2>/dev/null | json 'print(d["id"])' 2>/dev/null); then
+  id=$(curl -fsS -H "$auth" -H 'Content-Type: application/json' \
+    -d "{\"tag_name\":\"$tag\",\"name\":\"Das OS $version\",\"body\":\"Desktop and server live ISO images. Check them with sha256sum -c SHA256SUMS.\"}" \
+    "$api/releases" | json 'print(d["id"])')
+  echo "release: created $tag (id $id)"
+fi
+
+# Files left from an earlier run for this tag are replaced
+curl -fsS -H "$auth" "$api/releases/$id/assets" |
+  json '[print(a["id"], a["name"]) for a in d]' |
+  while read -r asset name; do
+    case " $isos SHA256SUMS " in
+      *" $name "*) curl -fsS -X DELETE -H "$auth" "$api/releases/$id/assets/$asset" ;;
+    esac
+  done
+
+for f in $isos SHA256SUMS; do
+  echo "release: uploading $f"
+  curl -fsS -o /dev/null -H "$auth" -F "attachment=@$f" "$api/releases/$id/assets?name=$f"
+done
+echo "release: $GITEA_SERVER_URL/$GITEA_REPOSITORY/releases/tag/$tag"