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/project-root.ts

@@ -0,0 +1,55 @@
+import { existsSync, statSync } from "node:fs";
+import { dirname, join } from "node:path";
+import { PROJECT_MARKERS } from "./constants";
+
+const projectRootCache = new Map<string, string | null>();
+
+export function clearProjectRootCache(): void {
+  projectRootCache.clear();
+}
+
+export function findProjectRoot(startPath: string): string | null {
+  const cached = projectRootCache.get(startPath);
+  if (cached !== undefined) return cached;
+  const startDir = resolveStartDir(startPath);
+  const cachedStartDir = projectRootCache.get(startDir);
+  if (cachedStartDir !== undefined) {
+    projectRootCache.set(startPath, cachedStartDir);
+    return cachedStartDir;
+  }
+
+  const visited: string[] = [];
+  let current = startDir;
+  let resolved: string | null = null;
+  while (true) {
+    const cachedAncestor = projectRootCache.get(current);
+    if (cachedAncestor !== undefined) {
+      resolved = cachedAncestor;
+      break;
+    }
+    visited.push(current);
+    if (hasProjectMarker(current)) {
+      resolved = current;
+      break;
+    }
+    const parent = dirname(current);
+    if (parent === current) break;
+    current = parent;
+  }
+
+  for (const directory of visited) projectRootCache.set(directory, resolved);
+  projectRootCache.set(startPath, resolved);
+  return resolved;
+}
+
+function resolveStartDir(startPath: string): string {
+  try {
+    return statSync(startPath).isDirectory() ? startPath : dirname(startPath);
+  } catch {
+    return dirname(startPath);
+  }
+}
+
+function hasProjectMarker(directory: string): boolean {
+  return PROJECT_MARKERS.some((marker) => existsSync(join(directory, marker)));
+}