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>

Makefile

@@ -40,5 +40,5 @@ test:
 
 # Build output may be owned by root when a build was interrupted, so remove it from the container
 clean:
-	scripts/builder.sh rm -rf /work/out/hostdir/binpkgs /work/out/iso
+	scripts/builder.sh rm -rf /work/out/hostdir/binpkgs /work/out/hostdir/stamps /work/out/iso
 	rm -rf out/qemu