feat(rules): add shared rules-core package

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

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

packages/rules-core/src/cache.ts

@@ -0,0 +1,26 @@
+import type { AgentsMdCache, DirectoryScanEntry, RuleFileCandidate, RuleScanCache, RuleScanCacheStats } from "./types";
+
+export function createRuleScanCache(): RuleScanCache {
+  const candidateCache = new Map<string, readonly RuleFileCandidate[]>();
+  const directoryCache = new Map<string, readonly DirectoryScanEntry[]>();
+  return {
+    get: (key) => candidateCache.get(key),
+    set: (key, value) => candidateCache.set(key, value),
+    getDirScan: (dir) => directoryCache.get(dir),
+    setDirScan: (dir, entries) => directoryCache.set(dir, entries),
+    stats: (): RuleScanCacheStats => ({ candidateEntries: candidateCache.size, directoryEntries: directoryCache.size }),
+    clear: () => {
+      candidateCache.clear();
+      directoryCache.clear();
+    },
+  };
+}
+
+export function createAgentsMdCache(): AgentsMdCache {
+  const cache = new Map<string, readonly string[]>();
+  return {
+    get: (key) => cache.get(key),
+    set: (key, value) => cache.set(key, value),
+    clear: () => cache.clear(),
+  };
+}