test(rules-core): isolate sisyphus deprecation warning assertion

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

packages/rules-core/src/finder.ts

@@ -10,6 +10,7 @@ import { log } from "../../../src/shared/logger";
 const SISYPHUS_DEPRECATION_MESSAGE = "[rules] .sisyphus/rules is deprecated and will be removed in v4.3.0; migrate to .omo/rules";
 const SISYPHUS_LEGACY_RULE_SOURCES: ReadonlySet<RuleSource> = new Set([".sisyphus/rules", "~/.sisyphus/rules"]);
 const warnedSisyphusRuleDirectories = new Set<string>();
+let logSisyphusRuleDeprecation: typeof log = log;
 
 export function findRuleFiles(
   projectRoot: string | null,
@@ -148,12 +149,21 @@ function warnSisyphusRuleDeprecation(source: RuleSource, path: string): void {
   const warningKey = dirname(path);
   if (warnedSisyphusRuleDirectories.has(warningKey)) return;
   warnedSisyphusRuleDirectories.add(warningKey);
-  log(SISYPHUS_DEPRECATION_MESSAGE, {
+  logSisyphusRuleDeprecation(SISYPHUS_DEPRECATION_MESSAGE, {
     event: "rules-sisyphus-deprecated",
     path,
   });
 }
 
+export function _setSisyphusRuleDeprecationLoggerForTesting(logger: typeof log): void {
+  logSisyphusRuleDeprecation = logger;
+}
+
+export function _resetSisyphusRuleDeprecationWarningStateForTesting(): void {
+  warnedSisyphusRuleDirectories.clear();
+  logSisyphusRuleDeprecation = log;
+}
+
 function validFileRealPath(filePath: string): string | null {
   if (!existsSync(filePath)) return null;
   try {