fix: remove unused fs imports in 3 hook scripts

readFile utility replaced direct fs usage but the imports weren't
removed, causing ESLint no-unused-vars failures in CI.

scripts/hooks/session-start.js

@@ -9,7 +9,6 @@
  * sessions and learned skills.
  */
 
-const fs = require('fs');
 const {
   getSessionsDir,
   getLearnedSkillsDir,