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/distance.ts

@@ -0,0 +1,25 @@
+import { dirname, relative } from "node:path";
+import { GLOBAL_DISTANCE } from "./constants";
+
+export function calculateDistance(rulePath: string, currentFile: string, projectRoot: string | null): number {
+  if (!projectRoot) return GLOBAL_DISTANCE;
+  try {
+    const ruleRelative = relative(projectRoot, dirname(rulePath));
+    const currentRelative = relative(projectRoot, dirname(currentFile));
+    if (ruleRelative.startsWith("..") || currentRelative.startsWith("..")) return GLOBAL_DISTANCE;
+    const ruleParts = toParts(ruleRelative);
+    const currentParts = toParts(currentRelative);
+    let shared = 0;
+    for (let index = 0; index < Math.min(ruleParts.length, currentParts.length); index += 1) {
+      if (ruleParts[index] !== currentParts[index]) break;
+      shared += 1;
+    }
+    return currentParts.length - shared;
+  } catch {
+    return GLOBAL_DISTANCE;
+  }
+}
+
+function toParts(path: string): string[] {
+  return path.split(/[/\\]/).filter(Boolean);
+}