refactor(packages): rename rules-core to rules-engine

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

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

packages/rules-engine/src/agents-md.ts

@@ -0,0 +1,50 @@
+import { existsSync, statSync } from "node:fs";
+import { dirname, isAbsolute, join, relative, resolve } from "node:path";
+import { AGENTS_FILENAME } from "./constants";
+import type { AgentsMdCache } from "./types";
+
+export interface FindAgentsMdUpInput {
+  readonly startDir: string;
+  readonly rootDir: string;
+  readonly skipRoot?: boolean;
+  readonly cache?: AgentsMdCache;
+}
+
+export async function findAgentsMdUp(input: FindAgentsMdUpInput): Promise<string[]> {
+  const startDir = resolve(input.startDir);
+  const rootDir = resolve(input.rootDir);
+  const skipRoot = input.skipRoot ?? true;
+  const cacheKey = [startDir, rootDir, skipRoot ? "1" : "0"].join("\0");
+  const cached = input.cache?.get(cacheKey);
+  if (cached) return [...cached];
+  const found: string[] = [];
+  let current = startDir;
+  while (true) {
+    const isRootDir = current === rootDir;
+    if (!(skipRoot && isRootDir)) {
+      const agentsPath = join(current, AGENTS_FILENAME);
+      if (isFile(agentsPath)) found.push(agentsPath);
+    }
+    if (isRootDir) break;
+    const parent = dirname(current);
+    if (parent === current || !isSameOrChildPath(parent, rootDir)) break;
+    current = parent;
+  }
+  const result = found.reverse();
+  input.cache?.set(cacheKey, result);
+  return result;
+}
+
+function isFile(path: string): boolean {
+  if (!existsSync(path)) return false;
+  try {
+    return statSync(path).isFile();
+  } catch {
+    return false;
+  }
+}
+
+function isSameOrChildPath(childPath: string, parentPath: string): boolean {
+  const relativePath = relative(parentPath, childPath);
+  return relativePath === "" || (!relativePath.startsWith("..") && !isAbsolute(relativePath));
+}