Refactor CLI to use flags instead of positional arguments

- Change all commands to use flags (--bucket, --prefix, etc.)
- Add --config flag to backup command
- Support VAULTIK_CONFIG environment variable for config path
- Use /etc/vaultik/config.yml as default config location
- Add test/config.yaml for testing
- Update tests to use environment variable for config path
- Add .gitignore for build artifacts and local configs
- Update documentation to reflect new CLI syntax

.gitignore

@@ -0,0 +1,22 @@
+# Binary
+vaultik
+
+# Test artifacts
+*.out
+*.test
+coverage.html
+coverage.out
+
+# IDE
+.vscode/
+.idea/
+*.swp
+*.swo
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Local config for development
+local-config.yaml
+dev-config.yaml