fix: make sessionExists() SQLite-aware for session_read tool

sessionExists() relied on JSON message directories which don't exist on
SQLite. Return true on SQLite and let readSessionMessages() handle lookup.
Also add empty-messages fallback in session_read for graceful not-found.

src/tools/session-manager/storage.ts

@@ -139,6 +139,7 @@ export function getMessageDir(sessionID: string): string | null {
 }
 
 export function sessionExists(sessionID: string): boolean {
+  if (isSqliteBackend()) return true
   return getMessageDir(sessionID) !== null
 }