refactor(rules): apply idiomatic patterns and resolve clippy warnings

- Change add_rule parameter from Option<&String> to Option<&str> for better ergonomics
- Use iterator methods (flat_map, any) instead of manual loops and collections
- Collapse nested if conditions using let-else patterns
- Use format string interpolation instead of separate arguments
- Avoid unnecessary String allocations in label comparisons
- Update CLI code to use as_deref() for Option<String> to Option<&str> conversion
- Update all documentation examples and tests to match new API
- All functionality preserved; API is more idiomatic and efficient

src/cli/rules_cli/config_cli/rules_cli/add_cli.rs

@@ -44,7 +44,7 @@ impl AddCli {
         let message_age = MessageAge::new(self.period.to_string().as_str(), self.count)?;
         let retention = Retention::new(message_age, generate);
 
-        config.add_rule(retention, self.label.as_ref(), self.delete);
+        config.add_rule(retention, self.label.as_deref(), self.delete);
         config.save()
     }
 }