chore(tmux-subagent): remove dead event-handler modules

Drop cleanup.ts, session-created-handler.ts, and session-deleted-handler.ts
which were never wired up; the lifecycle logic they contained lives inline
in TmuxSessionManager. Barrels trimmed to match.

src/features/tmux-subagent/event-handlers.ts

@@ -1,6 +1,2 @@
 export { coerceSessionCreatedEvent } from "./session-created-event"
 export type { SessionCreatedEvent } from "./session-created-event"
-export { handleSessionCreated } from "./session-created-handler"
-export type { SessionCreatedHandlerDeps } from "./session-created-handler"
-export { handleSessionDeleted } from "./session-deleted-handler"
-export type { SessionDeletedHandlerDeps } from "./session-deleted-handler"