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/index.ts

@@ -0,0 +1,20 @@
+export { createAgentsMdCache, createRuleScanCache } from "./cache";
+export { findAgentsMdUp, type FindAgentsMdUpInput } from "./agents-md";
+export { findRuleFiles } from "./finder";
+export { parseRuleFrontmatter } from "./parser";
+export { shouldApplyRule, createContentHash, isDuplicateByContentHash, isDuplicateByRealPath, resetMatcherCache, getMatcherCacheStats } from "./matcher";
+export { findProjectRoot, clearProjectRootCache } from "./project-root";
+export { calculateDistance } from "./distance";
+export { findRuleFilesRecursive, safeRealpathSync } from "./scanner";
+export type {
+  AgentsMdCache,
+  DirectoryScanEntry,
+  FindRuleFilesOptions,
+  MatchResult,
+  RuleFileCandidate,
+  RuleFrontmatterResult,
+  RuleMetadata,
+  RuleScanCache,
+  RuleScanCacheStats,
+  RuleSource,
+} from "./types";