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-file-scanner.ts

@@ -0,0 +1,55 @@
+import { existsSync, readdirSync, realpathSync } from "node:fs";
+import { join } from "node:path";
+import { GITHUB_INSTRUCTIONS_PATTERN, RULE_EXTENSIONS } from "./constants";
+
+function isGitHubInstructionsDir(dir: string): boolean {
+  return dir.includes(".github/instructions") || dir.endsWith(".github/instructions");
+}
+
+function isValidRuleFile(fileName: string, dir: string): boolean {
+  if (isGitHubInstructionsDir(dir)) {
+    return GITHUB_INSTRUCTIONS_PATTERN.test(fileName);
+  }
+  return RULE_EXTENSIONS.some((ext) => fileName.endsWith(ext));
+}
+
+/**
+ * Recursively find all rule files (*.md, *.mdc) in a directory
+ *
+ * @param dir - Directory to search
+ * @param results - Array to accumulate results
+ */
+export function findRuleFilesRecursive(dir: string, results: string[]): void {
+  if (!existsSync(dir)) return;
+
+  try {
+    const entries = readdirSync(dir, { withFileTypes: true });
+    for (const entry of entries) {
+      const fullPath = join(dir, entry.name);
+
+      if (entry.isDirectory()) {
+        findRuleFilesRecursive(fullPath, results);
+      } else if (entry.isFile()) {
+        if (isValidRuleFile(entry.name, dir)) {
+          results.push(fullPath);
+        }
+      }
+    }
+  } catch {
+    // Permission denied or other errors - silently skip
+  }
+}
+
+/**
+ * Resolve symlinks safely with fallback to original path
+ *
+ * @param filePath - Path to resolve
+ * @returns Real path or original path if resolution fails
+ */
+export function safeRealpathSync(filePath: string): string {
+  try {
+    return realpathSync(filePath);
+  } catch {
+    return filePath;
+  }
+}