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/project-root-finder.ts

@@ -0,0 +1,36 @@
+import { existsSync, statSync } from "node:fs";
+import { dirname, join } from "node:path";
+import { PROJECT_MARKERS } from "./constants";
+
+/**
+ * Find project root by walking up from startPath.
+ * Checks for PROJECT_MARKERS (.git, pyproject.toml, package.json, etc.)
+ *
+ * @param startPath - Starting path to search from (file or directory)
+ * @returns Project root path or null if not found
+ */
+export function findProjectRoot(startPath: string): string | null {
+  let current: string;
+
+  try {
+    const stat = statSync(startPath);
+    current = stat.isDirectory() ? startPath : dirname(startPath);
+  } catch {
+    current = dirname(startPath);
+  }
+
+  while (true) {
+    for (const marker of PROJECT_MARKERS) {
+      const markerPath = join(current, marker);
+      if (existsSync(markerPath)) {
+        return current;
+      }
+    }
+
+    const parent = dirname(current);
+    if (parent === current) {
+      return null;
+    }
+    current = parent;
+  }
+}