refactor(rules-injector): split finder.ts into rule discovery modules

Extract rule finding logic:
- project-root-finder.ts: project root detection
- rule-file-finder.ts: rule file discovery
- rule-file-scanner.ts: filesystem scanning for rules
- rule-distance.ts: rule-to-file distance calculation

src/hooks/rules-injector/rule-distance.ts

@@ -0,0 +1,53 @@
+import { dirname, relative } from "node:path";
+
+/**
+ * Calculate directory distance between a rule file and current file.
+ * Distance is based on common ancestor within project root.
+ *
+ * @param rulePath - Path to the rule file
+ * @param currentFile - Path to the current file being edited
+ * @param projectRoot - Project root for relative path calculation
+ * @returns Distance (0 = same directory, higher = further)
+ */
+export function calculateDistance(
+  rulePath: string,
+  currentFile: string,
+  projectRoot: string | null,
+): number {
+  if (!projectRoot) {
+    return 9999;
+  }
+
+  try {
+    const ruleDir = dirname(rulePath);
+    const currentDir = dirname(currentFile);
+
+    const ruleRel = relative(projectRoot, ruleDir);
+    const currentRel = relative(projectRoot, currentDir);
+
+    // Handle paths outside project root
+    if (ruleRel.startsWith("..") || currentRel.startsWith("..")) {
+      return 9999;
+    }
+
+    // Split by both forward and back slashes for cross-platform compatibility
+    // path.relative() returns OS-native separators (backslashes on Windows)
+    const ruleParts = ruleRel ? ruleRel.split(/[/\\]/) : [];
+    const currentParts = currentRel ? currentRel.split(/[/\\]/) : [];
+
+    // Find common prefix length
+    let common = 0;
+    for (let i = 0; i < Math.min(ruleParts.length, currentParts.length); i++) {
+      if (ruleParts[i] === currentParts[i]) {
+        common++;
+      } else {
+        break;
+      }
+    }
+
+    // Distance is how many directories up from current file to common ancestor
+    return currentParts.length - common;
+  } catch {
+    return 9999;
+  }
+}