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>

scripts/builder.sh

@@ -18,4 +18,5 @@ exec docker run --rm --privileged \
   --volume "$out/cache/xbps:/var/cache/xbps" \
   --env HOST_UID="$(id -u)" \
   --env HOST_GID="$(id -g)" \
+  --env DASOS_REBUILD \
   "$image" "$@"