Add CI, dirval-based validator, tests, and docs updates

• Add GitHub Actions workflow (Ubuntu, Python 3.10–3.12)
• Add Makefile (test + pkgmgr install note)
• Add requirements.yml (pkgmgr: dirval)
• Replace shell scripts with parallel validator main.py using dirval
• Add unit tests with fake dirval and timeouts
• Update README to new repo name and pkgmgr alias 'cleanback'
• Add .gitignore for __pycache__

Conversation context: https://chatgpt.com/share/68c309bf-8818-800f-84d9-c4aa74a4544c

Makefile

@@ -0,0 +1,18 @@
+# Makefile for Cleanup Failed Backups
+
+.PHONY: test install help
+
+help:
+	@echo "Available targets:"
+	@echo "  make test     - Run unit tests"
+	@echo "  make install  - Show installation instructions"
+
+test:
+	@echo ">> Running tests"
+	@python3 -m unittest -v test.py
+
+install:
+	@echo ">> Installation instructions:"
+	@echo "   This software can be installed with pkgmgr:"
+	@echo "     pkgmgr install cleanback"
+	@echo "   See project: https://github.com/kevinveenbirkenbach/package-manager"