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/index.ts

@@ -1 +1,2 @@
 export { createRulesInjectorHook } from "./hook";
+export { calculateDistance, findProjectRoot, findRuleFiles } from "./finder";