fix: use _resetForTesting() consistently to prevent flaky tests (#1318)

- Replace setMainSession(undefined) with _resetForTesting() in keyword-detector tests
- Add _resetForTesting() to afterEach hooks for proper cleanup
- Un-skip the previously flaky mainSessionID test in state.test.ts

Fixes #848

Co-authored-by: 배지훈 <new0126@naver.com>

src/hooks/session-notification.test.ts

@@ -45,7 +45,7 @@ describe("session-notification", () => {
   afterEach(() => {
     // #given - cleanup after each test
     subagentSessions.clear()
-    setMainSession(undefined)
+    _resetForTesting()
   })
 
   test("should not trigger notification for subagent session", async () => {